GIFT  OF 
Fred  Whitworth 


Engineering  Library 
1939 


PROTECTIVE  RELAYS 


PUBLISHERS     OF    BOOKS     F  O  FO 

Hectrical  WDrld  v  Engineering  News-Record 
Power  v  Engineering  and  Mining  Joumal-ftess 
Chemical  and  Metallurgical  Engineering 
Electric  Railway  Journal  v  Coal  Age 
American  Machinist v  Ingenieria  Internacional 
Hectrical  Merchandising  v  BusTransportation 
Journal  of  Electricity  and  Western  Industry 
Industrial  Engineer 


PROTECTIVE  RELAYS 


THEIR  THEORY,  DESIGN, 

AND 
PRACTICAL    OPERATION 


BY 


VICTOR  H.  TODD 

DESIGNING  AND  MANUFACTURING  ELECTRICAL,  ENGINEER 

WE8TINQHOU8E   ELECTRIC  &   MANUFACTURING    CO. 

MEMBER  A.  I.  E.   E. 


FIRST  EDITION 
THIRD  IMPRESSION 


McGRAW-HILL  BOOK  COMPANY,  INC. 
NEW  YORK:  370  SEVENTH  AVENUE 

LONDON:  6  &  8  BOUVERIE  ST.,  E.  C.  4 

1922 


Engineering 
Xibrary 


ENGINEERING  LIBRARY 


COPYRIGHT,  1922,  BY  THE 
MCGRAW-HILL  BOOK  COMPANY,  INC. 

PRINTED  I  N*THE  UNFTEB  STATES  OF  AMERICA 


THE  MAPLE  PRESS  -  YORK  PA 


PREFACE 

In  preparing  a  work  of  this  nature,  it  will  be  realized  that  the 
subject  is  so  broad  and  varied  in  its  scope  as  to  preclude  an 
author's  intimate  personal  knowledge  of  each  and  every  system 
described.  Attempt  has  been  made  to  cover  the  subject  from 
the  first  principles  of  Protective  Relays  to  the  protection  of 
high  tension  net-works,  the  object  being  to  make  the  work  of 
value  not  only  to  the  operator  and  tester  who  has  a  fair  knowledge 
of  electricity  and  is  seeking  more  information,  but  also  to  the 
designer  of  the  system  who  may  find  many  points  not  previously 
taken  into  consideration  in  his  calculations. 

Consequently,  many  reliable  sources  of  information  were 
freely  consulted  and  in  several  instances,  passages  were  quoted 
verbatim  from  a  booklet  entitled  "Performance  of  Instrument 
Transformers"  and  from  " Protective  Relays"  which  in  turn 
was  reprinted  from  an  article  by  Mr.  L.  N.  Crichton  in  The 
Electric  Journal.  Full  credit  is  hereby  given  to  the  Westing- 
house  Electric  &  Manufacturing  Company  for  such  excerpts; 
also  to  the  American  Institute  of  Electrical  Engineers  for  several 
paragraphs  quoted  from  its  June,  1919,  Proceedings. 

Some  of  this  material  has  been  published  in  the  form  of  articles 
by  the  author  in  Power,  Power  Plant  Engineering,  Electrical  Record 
and  Southern  Engineer,  and  thanks  are  hereby  given  to  their 
editors  for  permission  to  use  the  text  and  electro-types  for 
illustrations. 

The  author  also  wishes  to  extend  his  thanks  and  appreciation 
to  Mr.  F.  A.  Annett,  Associate  Editor  of  Power y  for  personal 
interest  and  assistance  in  the  preparation  of  the  work;  to  Mr. 
L.  N.  Crichton,  Relay  Engineer  of  the  Westinghouse  Electric 
&  Manufacturing  Company,  and  writer  of  several  articles 
from  which  much  valuable  information  was  obtained,  who 
kindly  read  this  manuscript,  and  offered  many  helpful  sugges- 
tions and  constructive  criticisms;  and  to  officials  of  the  Westing- 
house  Electric  &  Manufacturing  Company,  the  General  Electric 
Company  and  the  Condit  Electric  Company,  for  valuable  assis- 
tance in  procuring  photographs  for  illustrations. 

VICTOR  H.  TODD. 

SUMMIT,  N.  J.,  ^. 

December,  1921.  M36818 

vii 


CONTENTS 

PAGE 

PREFACE V 

CHAPTER  I 

WHAT  ARE  PROTECTIVE  RELAYS? 1 

Early  Systems — Fire  Risks — Insulated  Fuses — Objections — Auto- 
matic Switches — Definition — Reasons  for  Relays — Requirements — 
Savings — Principles  of  Operation — Nomenclature. 

CHAPTER  II 

CIRCUIT  BREAKERS  AND  RELEASES 7 

Classes — Principles  of  Operation — Adjustment — Shunt-trip 
Attachment — Underload  Release — Undervoltage  Release — Over- 
voltage  Release — Why  Releases  are  not  Always  Sufficient. 

CHAPTER  III 

TRIP  CIRCUITS  AND  TIME  DELAYS 16 

Object  of  Trip — Trip  Circuit  Sources — Shunt  Trip — Auxiliary 
Contacts — Circuit  Opening  or  Series  Trip — Objections — Transfer 
Relays — Time  Delays — Classifications — Inverse-time  Limit — 
Definite  Time — Inverse-definite  Time — How  Time  Delays  are 
Obtained. 

CHAPTER  IV 

PLUNGER-TYPE  PROTECTIVE  RELAYS 24 

Various  Forms — Adjustments — More  Forms — Definite-time 
Relay  —  Bellows-type  —  Objections  —  Simple  System  —  Typical 
A.C.  Plunger-type — Illustration  of  Setting — Relays  Required — 
More  Bellows  Types — Oil  Dashpot  Types — Limitations — Definite- 
time — Tripping  Sources. 

CHAPTER  V 

D.C.  POWER-DIRECTIONAL  RELAYS 44 

Necessity  of  this  Type — D'Arsonval  Type — Used  as  Excess- 
current  Relay — Polarized  Type — Moving  Iron  Type — Busbar 
Types. 

ix 


x  CONTENTS 

CHAPTER  VI 

PAGE 
APPLICATIONS  OF  D.C.  POWER-DIRECTIONAL  RELAYS 55 

Limitations  to  Use — Storage  Battery  Protection — Other  Methods 
— Standby  Batteries — Failure  of  Prime  Movers — Loss  of  Field  Pro- 
tection— Parallel  Feeders  on  D.C. — Ring  System  on  D.C. — Wider 
Applications — Undercurrent  Protection — Overvoltage  Protection 
— Undervoltage  Protection. 

CHAPTER  VII 

INDUCTION-TYPE  CURRENT  RELAYS 66 

Development — Typical  Relay — Settings — Torque  Compensator — 
Time  Delays — Continuity  of  Trip  Indicator — Induction  vs.  Sole- 
noid-plunger Relays — Relay  Contacts — Load  on  Transformer — 
Relay  Specifications. 

CHAPTER  VIII 

A.C.  POWER-DIRECTIONAL  RELAYS 85 

Early  development — Most  Common  Use — Overload  and  Reverse- 
current  Relays — Objections  and  Failings — Latest  Developments — 
The  Contactor  Switch — The  Torque  Compensator — Stray  Field 
Effect — Other  Types — Differential  Power-directional  Relay. 

CHAPTER  IX 

CHARACTERISTICS  OF  A.C.  DISTURBANCES  . 98 

Important  Points — Effects  of  Overload — Nature  of  Short-circuits 
on  Transmission  Lines — Calculation  of  the  Short-circuit  Current 
— Alternator  and  Transformer  Constants — Effect  of  Low  Voltage 
— Effect  of  Unbalanced  Short-circuits — Characteristics  of  Relays. 


CHAPTER  X 

INSTRUMENT  TRANSFORMERS  AND  GROUPINGS 112 

Current  and  Voltage  Transformers — Inherent  Errors — Ratio  Error 
— Magnetization  of  Core — Effect  of  Secondary  Load — Other 
Sources  of  Error — Single-phase  Groupings — Two-phase  Groupings 
— Necessity  for  Three  Transformers  on  Three-phase — Advantages 
of  the  Z-connection — Opening  of  Secondary — Voltage  Trans- 
formers— Various  Errors — Polyphase  Groupings — Load  on  Trans- 
formers— Use  of  Both  Current  and  Potential  Transformers — 
Connections  to  Watt  Relay — Star-delta  Connection — Determin- 
ing Phase  Rotation. 


CONTENTS  xi 

CHAPTER  XI 

PAGE 

PROTECTION  OF  MOTORS,  TRANSFORMERS,  GENERATORS  AND  LINES  .  .  132 
Protection  of  Motors — Settings — Two-phase  Protection — Three- 
phase  Protection — Protection  of  Synchronous  Motors — Protec- 
tion of  Rotary  Converters — Transformer  Protection — Protection 
in  Banks — Power-directional  Relay  Protection — Other  Differential 
Methods — Polyphase  Transformer  Protections — Protecting  Three- 
phase  Star-delta  Banks — Protection  of  Generators — Protection  by 
Power-directional  Relays — Protection  of  Single  Lines — Protection 
Against  Grounds. 

CHAPTER  XII 

PROTECTION  OF  PARALLEL  FEEDERS 152 

Objects — Various  Methods — Inverse-time-limit  Discrimination 
— Balanced  Protection  System — Differential  Balance  Relay 
Protection — Split-conductor  System — The  Pilot  Wire  System — 
Protection  by  Power-directional  Relays — Cross-connected  Power- 
directional  Relays — Differential  Power-directional  or  Double- 
contact  Relays — Disadvantages  of  Cross-connected  Systems. 

CHAPTER  XIII 

PROTECTION  OF  RADIAL  RING  AND  NETWORK  SYSTEMS 175 

Simple  Radial  System — Selecting  the  Proper  Relay — Limit  of 
Divisions — The  Ring  System — Time  Settings — Parallel  Feeders 
on  Ring — Rings  with  More  than  One  Source — Protection  of 
Networks — The  Undervoltage  and  Excess-current  System. 

CHAPTER  XIV 

MISCELLANEOUS  RELAYS 189 

Over  and  Undervoltage — Undercurrent — Overload  Telegraph — 
Reversed  Phases — Service-restoring  Relays — Interesting  Oscillo- 
grams — Bell-ringing  Relays — D.C.  Temperature  Relays — A.C. 
Temperature  Relays — Relay  Switches — Transfer  Relays — High- 
tension  Relays — Timing  Relays  with  a  Cycle  Counter — Princi- 
ple of  Cycle  Counter — Timing  a  Circuit-closing  Relay — Timing  a 
Circuit-opening  Relay — Timing  the  Breaker  or  Oil  Switch — 
Typical  Layout. 

CHAPTER  XV 

TESTING  DIRECT-CURRENT  RELAYS 220 

Ground-testing — Testing  Relay  Switches — Various  Loads  and 
Testing  Sources — Testing  Millivolt-type  Relays — Testing  Plun- 
ger-type Relays — Testing  Time-limit  Relays — Curves  and 
Tables — Conclusions . 


xii  CONTENTS 

CHAPTER  XVI 

PAGE 

TESTING  ALTERNATING-CURRENT  RELAYS 231 

Reasons  for  Testing — Relays  Requiring  Current  Only — Sources — 
Rheostats — Phantom  Loads — Standards — Current  Transformers 
—Trip  Circuits— Timing  the  Relay— An  Actual  Test— The  Cycle 
Counter — Making  the  Adjustment — Additional  Precautions — 
Testing  Voltage  Relays — Power-directional  Relays — Reverse- 
phase  Relays — Temperature  Relays — Conclusion. 

CHAPTER  XVII 

LOCATING  FAULTS  IN  FEEDERS  AND  WIRING 251 

Most  Common  Faults — Apparatus  Required — Testing  for  Opens, 
Shorts  or  Grounds — Accurately  Locating  the  Short-circuit — 
Localizing  a  Ground — Calculating  the  Location — Two-ammeter 
Method — The  Fault  Localizer — L  and  N  Power  Bridge — Burning 
out  the  Fault. 

INDEX  .  .  265 


PROTECTIVE  RELAYS 

CHAPTER    I 

WHAT    ARE    PROTECTIVE     RELAYS 

When  the  first  electric  generators  were  built  and  installed, 
no  provision  was  made  for  disconnecting  them  automatically 
from  the  lines  in  case  of  trouble  such  as  overload  or  short- 
circuits.  This  was  overlooked  for  two  reasons:  (1)  It  was  not 
considered  detrimental  to  have  service  interrupted,  as  the 
electric  system  was  more  or  less  a  novelty  and  in  the  ex- 
perimental stage,  and  (2)  the  design  of  generators  and  prime 
movers  was  such  that  in  case  of  heavy  overload  either  the 
prime  mover  would  stop,  or  the  belt  fly  off,  or  the  voltage  drop 
so  low  that  no  electrical  damage  could  be  done. 

However,  as  the  mechanical  and  electrical  design  of  genera- 
tors and  prime  movers  was  gradually  improved,  it  was  noticed 
that  in  the  event  of  a  heavy  overload,  the  increased  current 
would  burn  out  the  weakest  spot  in  the  system;  sometimes 
this  was  in  the  armature,  sometimes  in  the  wiring  and  sometimes 
in  the  switches.  Naturally  the  result  was  to  insert  intentionally 
a  weak  spot  at  some  convenient  point  in  the  system,  by  con- 
necting a  piece  of  wire  much  smaller  than  the  main  wires,  so 
that  when  the  overload  occurred,  this  wire  would  burn  out 
and  then  the  operator  knew  just  where  to  go  and  look  for  trouble 
when  the  power  went  off.  This  was  the  first  fuse. 

Installing  the  fine  wiring  near  inflammable  material  led  to 
another  trouble;  i.e.,  fire  risk,  as  during  a  heavy  short-circuit, 
the  wire  was  melted,  and  the  red-hot  metal  thrown  violently 
in  all  directions. 

The  remedy  was  to  enclose  the  fuse  in  an  insulating  non- 
inflammable  tube  to  prevent  fires,  and  also  to  provide  fuse  blocks 
and  clips  so  that  the  fuses  could  be  replaced  easily  in  case  of 
an  overload  blowing  them  out. 

1 


2  PROTECTIVE  RELAYS 

Even  today,  there  is  no  other  piece  of  apparatus  which 
can  exc-ei  p.  properly  designed  fuse  for  reliability,  and  no  matter 
what  other  protective  apparatus  is  installed,  as  will  be  described 
later,  there  is  hardly  an  installation  which  does  not  have  fuses 
as  an  absolute  guarantee  of  protection  against  heavy  overloads, 
should  the  other  protective  apparatus  fail. 

Objections. — The  greatest  objection  to  fuses  was  the  main- 
tenance cost,  or  cost  of  replacement,  as  every  time  a  fuse  blew 
out,  it  meant  a  new  fuse,  and  as  the  current  capacity  of  the  system 
went  up,  every  new  fuse  meant  considerable  money.  Not 
only  that,  but  it  took  some  time  to  locate  and  replace  a  fuse 
and  besides,  if  a  new  fuse  were  not  immediately  at  hand,  there, 
was  a  severe  temptation  to  use  a  convenient  piece  of  copper  or 
solder-wire  to  complete  the  circuit,  thus  again  introducing  a 
a  hazard.  This  improper  replacement  of  fuses  is  one  of  the 
deepest-rooted  evils  in  the  electrical  industry  and  is  alone 
responsible  for  thousands  upon  thousands  of  dollars'  worth 
of  damage  by  fire  every  year,  in  addition  to  the  burning  out 
of  many  motors,  generators  and  other  apparatus. 

In  order  to  eliminate  this  replacement  expense,  as  well  as 
to  restore  service  in  a  minimum  of  time,  it  was  found  necessary 
to  develop  an  automatic  switch  which  would  open  the 
circuit  in  the  event  of  trouble.  This  was  the  forerunner 
of  the  present  circuit  breaker.  There  were  many  types  de- 
veloped and  successfully  used,  but  the  principles  and  the  objects 
accomplished  were  all  the  same;  a  coil  carrying  the  main  current 
was  arranged  to  either  attract  an  armature  fastened  to  the 
switch  blade  and  thus  open  the  switch  directly,  or  to  attract 
an  armature  which  in  turn  released  a  spring,  thus  allowing 
the  spring  to  open  the  switch. 

From  the  foregoing  it  would  appear  that  the  problem  of  pro- 
tection was  solved.  But  protection  was  not  the  only  desirable 
feature;  customers  demanded  continuous  service  and  many 
times  an  interruption  was  caused  only  by  a  transient  short- 
circuit;  for  instance,  a  wire  or  tool  dropped  across  a  switch, 
immediately  falling  off  or  burning  out.  It  was  not  necessary 
for  the  breaker  to  open  as  the  overload  disappeared  almost 
immediately  and  if  the  breaker  had  not  opened,  service  would 
have  continued  uninterrupted.  In  other  words,  the  circuit 


WHAT  ARE  PROTECTIVE  RELAYS  3 

breaker  had  no  discriminating,  " reasoning"  or  " thinking" 
power.  This  led  to  the  development  of  the  protective  relay. 

Protective  Relays. — The  protective  relay  is  an  electrical 
instrument,  generally  accurate,  sensitive,  rugged  and  reliable 
in  construction,  interposed  between  the  main  circuit  and  the 
circuit  breaker  in  such  a  manner  that  any  abnormality  in  the 
circuit  acts  on  the  relay,  which  in  turn,  after  the  proper  dis- 
crimination of  the  magnitude  and  characteristic  of  the  ab- 
normality, causes  other  apparatus  such  as  a  circuit  breaker 
to  function  and  relieve  or  protect  the  circuit  and  apparatus. 

The  great  need  of  adequate  protection  and  continuous  service 
under  all  conditions  has  gradually  caused  the  crude  types  of 
relays  of  a  few  years  ago,  which  sometimes  gave  protection  under 
special  conditions  but  often  failed  at  the  critical  moment, 
to  be  developed  into  present-day  types,  which  are  built  with 
the  accuracy  of  a  watt-hour  meter  and  can  be  depended  upon 
in  practically  all  conceivable  cases  of  electrical  distress. 

In  alternating-current  systems,  the  need  of  continuity  of 
service  is  so  great  that  the  lines  must  be  kept  alive  until  there 
is  no  chance  of  the  disturbance  clearing  itself.  In  some  states 
the  public  utility  boards  require  an  explanation  of  each  and  every 
interruption  that  occurs  in  an  operating  company's  service 
as  well  as  a  report  of  the  steps  taken  to  prevent  its  recurrence. 

Many  power  plants  and  factory  managements  have  required 
in  the  past  duplicate  supply  lines,  so  that  their  power  supply 
would  not  be  interrupted  in  case  of  trouble  on  other  parts  of 
the  system.  Others  have  maintained  stand-by  plants  ready 
to  assume  the  load  in  case  of  electrical  trouble. 

As  a  typical  example  of  how  protective  relays  have  eliminated 
this  necessity,  one  operating  company  had  as  many  as  25  in- 
terruptions a  year,  but  after  making  an  intelligent  survey  of 
the  system,  a  few  changes  were  made  in  the  sectionalizing 
apparatus  and  accurate  relays  installed  which  reduced  the 
interruptions  to  about  one  annually,  although  the  system 
suffered  not  less  than  100  short-circuits  per  year  from  various 
causes. 

The  relays  instantly  sectionalize  and  isolate  a  defective 
line  or  piece  of  apparatus  without  disturbing  the  rest  of  the 
system.  This  allows  spare  lines  to  be  used  continually,  and  the 


4  PROTECTIVE  RELAYS 

great  saving  and  economy  of  copper  will   often   finance  the 
installation  of  relays. 

Like  a  silent  sentinel,  the  protective  relay  stands  on  guard 
on  the  lines,  day  and  night,  summer  and  winter,  ready  to 
detect  trouble  instantly,  to  determine  if  it  is  serious  enough 
to  open  the  circuit,  and,  if  so,  to  disconnect  faulty  apparatus 
or  sectionalize  defective  lines  with  almost  human  intelligence 
and  more  than  human  accuracy. 

At  present,  the  protective  relays  are  so  specialized  and  highly 
developed  that  there  is  practically  no  electrical  defect  or  ab- 
normal condition  on  a  line  that  cannot  be  detected  by  a  relay 
and  the  circuit  protected  against  it.  Excess  current  (generally 
called  "overload"),  under  current,  over- voltage,  under- voltage, 
over  and  under-wattage,  reverse  current  or  power,  high  or  low 
frequency,  high  or  low  temperature,  reverse  phases,  and  numer- 
ous other  conditions  which  may  occur,  all  can  be  detected  by 
a  suitable  relay. 

Principles  of  Operation. — As  electricity  is  an  intangible  some- 
thing which  cannot  be  measured  like  water  or  gas,  we  must 
detect  its  presence  and  characteristics  by  the  effect  it  produces, 
The  effects  are  four  in  number:  chemical  changes,  heat,  magnet- 
ism and  static  attraction.  Although  electrical-indicating 
instruments  have  been  made  to  operate  on  all  these  various 
effects,  yet  practically  all  protective  relays  depend  on  the  mag- 
netic effect  of  an  electric  current  for  their  operation. 

The  three  main  principles  used  are  (1)  the  D'Arsonval  prin- 
ciple, utilizing  a  moving  coil  reacting  on  a  permanent  magnet: 
(2)  the  solenoid  and  plunger  type,  utilizing  the  sucking  effect 
of  an  energized  solenoid  on  an  iron  plunger;  and  (3)  the  induc- 
tion type,  utilizing  the  same  principle  as  employed  in  induc- 
tion motors  and  watt-hour  meters.  Various  relays  operating 
on  these  principles  will  be  considered  in  detail  on  following 
pages. 

NOMENCLATURE 

As  relay  development  has  been  a  gradual  evolution,  many 
firms  developing  a  certain  relay  to  overcome  their  particular 
troubles,  it  will  be  readily  apparent  why  there  should  be  such 
a  wide  variation  in  the  nomenclature  throughout  the  country. 


WHAT  ARE  PROTECTIVE  RELAYS  5 

In  an  effort  to  standardize  and  harmonize  the  nomenclature 
of  protective  relays  the  Protective  Devices  Committee  of  the 
A.  I.  E.  E.  recommended  the  following  terms: 

Protective  Relay. — An  intermediate  electrical  instrument 
by  means  of  which  one  circuit  is  indirectly  controlled  by  a  change 
in  conditions  in  the  same  or  other  circuits.  The  relay  is  gener- 
ally equipped  with  contacts  which  are  closed  or  opened  mechan- 
ically by  a  change  on  one  circuit  and  these  contacts  in  turn 
close  or  open  an  auxiliary  circuit  electrically. 

Directional  Relay. — Any  relay  which  functions  in  conform- 
ance  with  the  direction  of  power,  or  voltage  or  current  or  phase 
rotation,  etc. 

Power -directional  Relay. — Any  relay  which  functions  in  con- 
formance  with  direction  of  power.  This  includes  both  uni- 
directional relays  with  single  contacts  and  duo-directional 
relays  with  double  contacts.  This  term  is  to  be  preferred 
to  "  re  verse  power"  or  "  re  verse  current"  relays  because  this 
type  is  frequently  used  to  function  under  normal  direction 
of  power.  Furthermore,  in  some  cases,  the  normal  condition 
of  the  system  may  permit  power  to  flow  in  either  direction. 

Polarity-directional  Relay. — Any  relay  which  functions  by 
reason  of  change  in  rjolarity. 

Phase -rotation  Relay. — Any  relay  which  functions  by  reason 
of  a  reversal  of  the  normal  direction  of  phase  rotation. 

Current  Relay. — Any  relay  which  functions  at  a  predetermined 
value  of  the  current.  These  may  be  either  over-current  relays 
or  under-current  relays  and  are  commonly  called  " overload" 
and  " underload"  relays. 

Voltage  Relay. — Any  relay  which  functions  at  a  predetermined 
value  of  the  voltage.  These  may  be  either  over- voltage  relays 
or  under-voltage  relays. 

Watt  Relay. — Any  relay  which  functions  at  predetermined 
value  of  the  watts.  These  may  be  either  over-watt  relays 
or  under-watt  relays. 

Frequency  Relay. — Any  relay  which  functions  at  predeter- 
mined value  of  the  frequency.  These  may  be  either  over- 
frequency  relays  or  under-frequency  relays. 

Temperature  Relay. — Any  relay  which  functions  at  a  pre- 
determined temperature  in  the  apparatus  protected. 


6  PROTECT  I VE  RELA  YS 

Open-phase  Relay. — Any  relay  which  functions  by  reason 
of  the  opening  of  one  phase  of  a  polyphase  circuit. 

Differential  Relay. — Any  relay  which  functions  by  reason 
of  the  difference  between  two  quantities  such  as  current  or 
voltage,  etc.  This  term  includes  relays  heretofore  known  as 
"ratio-balance  relays,"  "biased"  and  "percentage-differential 
relays." 

Locking  Relay. — Any  relay  which  renders  some  other  relay 
or  other  device  inoperative  under  predetermined  values  of 
current  or  voltage,  etc. 

Trip-free  Relay. — Any  relay  which  prevents  holding  in  an 
electrically-operated  device  such  as  a  circuit  breaker  while  an 
abnormal  condition  exists  on  the  circuit. 

Auxiliary  Relay. — Any  relay  which  assists  another  relay  in 
the  performance  of  its  function  and  which  operates  in  response 
to  the  opening  or  closing  of  its  operating  circuit.  Sometimes 
called  "relay  switches,"  "contactors"  or  "multicontact  relays." 

Signal  Relay. — An  auxiliary  relay  which  operates  an  audible 
or  a  visible  signal.  Sometimes  called  "bell  ringing  relays." 

QUALIFYING  TERMS  AS  APPLIED  TO  RELAYS 

Notching. — A  qualifying  term  applied  to  any  relay  indicat- 
ing that  a  number  of  separate  impulses  are  required  to  complete 
operation. 

Inverse  Time. — A  qualifying  term  applied  to  any  relay 
indicating  that  there  is  purposely  introduced  a  delayed  action, 
which  delay  decreases  as  the  operating  force  increases. 

Definite  Time. — A  qualifying  term  applied  to  any  relay 
indicating  that  there  is  purposely  introduced  a  delayed  action, 
which  delay  remains  substantially  constant  regardless  of  the 
magnitude  of  the  operating  force.  (For  forces  slightly  above 
the  minimum  operating  value  the  delay  may  be  inverse). 

Instantaneous. — A  qualifying  term  applied  to  any  relay 
indicating  that  no  delayed  action  is  purposely  introduced. 

Where  relays  operate  in  response  to  changes  in  more  than 
one  condition,  all  functions  should  be  mentioned. 


CHAPTER    II 

CIRCUIT  BREAKERS  AND  RELEASES 

In  order  to  appreciate  fully  the  action  of  the  protective 
relays,  it  is  first  necessary  to  understand  the  action  of  circuit 
breakers,  both  how  they  may  be  equipped  with  certain  releases 
which  operate  on  abnormal  conditions  such  as  over  and  under 
current,  etc.,  and  also  how  they  may  be  equipped  with  trip 
coils  which  are  actuated  electrically  by  protective  relays. 

As  previously  stated,  the  first  circuit  breakers  were  merely 
automatic  switches,  arranged  with  an  electromagnet  to  cause 
the  blades  to  fly  open  upon  the  occurrence  of  excess  current 
in  the  electromagnet.  Present-day  circuit  breakers  break  the 
circuit  either  in  air  or  under  oil.  The  first  are  commonly  called 
"carbon  circuit  breakers"  because  the  final  break  is  between 
carbon  contacts,  and  the  second  kind  are  often  called  "automatic 
oil  switches,"  or  "oil  circuit  breakers."  Breakers  may  be 
semi-automatic,  fully-automatic  or  electrically-operated.  A 
semi-automatic  circuit  breaker  is  one  which  opens  automati- 
cally on  the  occurrence  of  an  abnormal  condition,  but  must  be 
closed  by  hand,  and  if  the  handle  is  held  in  a  closed  position, 
the  breaker  is  inoperative.  In  the  fully-automatic  breaker, 
the  handle  is  said  to  be  "trip-free;"  that  is,  if  the  breaker 
has  opened  due  to  an  abnormal  condition,  and  attempt  is  made 
to  re-close  the  breaker  by  hand,  then  if  the  abnormal  condition 
still  exists,  the  breaker  will  open  even  though  the  handle  be 
held  in  the  closed  position.  An  electrically-operated  breaker 
is  one  which  is  both  closed  and  opened  electrically.  The 
opening  is  usually  due  to  an  abnormal  condition,  but  the  breaker 
may  be  either  opened  or  closed  from  some  remote  source,  for 
instance  a  remote-control  panel  board  or  a  protective  relay. 

PRINCIPLE  OF  OPERATION 

Circuit  breakers  are  usually  arranged  so  that  when  they 
are  closed,  considerable  energy  is  stored  in  strong  springs, 
and  the  parts  held  in  a  closed  position  by  a  small  trigger  or 

7 


8  PROTECTIVE  RELAYS 

tripping  lever.  When  the  predetermined  conditions  occur 
for  which  the  breaker  is  set,  or  when  actuated  by  the  protec- 
tive relay,  this  trigger  is  tripped,  thereby  releasing  the  poten- 
tial energy  of  the  springs  which  causes  the  contacts  to  open 
in  a  small  fraction  of  a  second. 

This  may  be  seen  readily  from  the  schematic  diagram  of  parts 
of  a  typical  carbon  circuit  breaker  as  shown  in  Fig.  2,  which 
shows  the  breaker  in  its  normally  closed  position.  A  and 
B  are  the  main  contacts  (usually  heavy  copper  blocks),  which 
are  spanned  by  the  laminated  copper  brush  C.  This  is  loosely 
attached  to  the  moving  arm  D,  in  such  a  manner  that  it  is 
self-aligning  and  when  the  breaker  is  being  closed,  contact 
C  is  forced  against  contacts  A  and  B  with  great  pressure,  re- 
sulting in  a  " wiping"  action  which  makes  a  good  electrical 
connection.  The  moving  arm  D  is  pivoted  at  E  and  the  springs 
F  are  under  tension,  tending  to  pull  the  contact  open.  This 
is  prevented  by  the  toggle  joint  G  and  H  which  is  held  in 
position  by  the  trigger  I  (pivoted  at  J). 

Connected  between  the  lower  stud  on  the  breaker,  and  the 
lower  main  contact,  is  a  coil  K  which  carries  the  main  current; 
and  under  its  influence  is  an  iron  armature  L  pivoted  at  M. 
When  the  current  in  coil  K  becomes  great  enough,  the  armature 
L  is  attracted  to  its  core;  this  causes  a  projection  on  the  armature 
to  strike  the  tripping  lever  a  sharp  blow,  thereby  releasing 
the  toggle  joint  and  allowing  the  springs  to  open  the  contacts 
quickly.  In  Fig.  3  is  shown  the  breaker  just  at  the  instant 
of  tripping  and  Fig.  4  shows  the  breaker  fully  open. 

In  order  to  reset,  the  handle  S  must  be  pulled  down, 
thereby  straightening  the  toggle  joint,  extending  the  springs 
and  giving  the  necessary  power  to  force  the  contacts  into 
tight  contact. 

If  the  main  contacts  were  to  open  the  circuit  directly,  the 
resultant  arcing  would  soon  cause  them  to  become  so  pitted 
that  they  could  no  longer  make  contact.  Therefore,  the  circuit 
is  finally  broken  by  two  auxiliary  carbon  contacts  at  N  and  0; 
when  the  main  contacts  open,  the  current  is  shunted  through 
these  contacts,  but  they  are  separated  so  quickly  that  no  arc 
or  excessive  burning  results,  the  oxide  of  course  passing  off  as 
a  gas. 


CIRCUIT  BREAKERS  AND  RELEASES 


9 


10  PROTECTIVE  RELAYS 

In  order  to  allow  a  variation  in  the  tripping  current  required, 
the  armature  L  is  provided  with  a  threaded  weight  P  which 
is  mounted  on  a  worm  Q  and  so  its  position  may  be  varied  by 
turning  the  nut  R.  When  the  weight  is  near  the  electromagnet 
K  its  full  weight  tends  to  keep  the  armature  away  from 
the  electromagnet,  thus  requiring  the  maximum  current  to 
cause  tripping;  when  it  is  near  the  nut  R  its  weight  tends  to 
assist  the  attraction  of  K,  thereby  requiring  a  minimum  of  current. 

Provision  is  usually  made  to  open  the  breakers  by  hand. 
This  breaker  is  opened  by  raising  the  handle  S,  which  results 
in  tripping.  Many  operators  prefer  to  touch  the  trip  lever 
7,  or  the  nut  R,  to  open  the  breaker,  but  this  should  be  done 
with  great  caution  as  this  part  is  "alive"  and  there  may  be 
danger  of  severe  shock  by  touching.  Many  breakers  provide 
for  opening  in  this  manner  by  using  a  well-insulated  nut  at 
R,  thus  preventing  danger  of  accidental  shock. 

Shunt -trip  Attachment. — It  will  be  readily  seen  that  anything 
which  trips  the  lever  I  will  cause  the  breaker  to  open.  The  shunt 
trip  is  a  device  for  tripping  the  breaker,  from  some  remote  source, 
either  manually  or  automatically  by  a  protective  relay.  In 
Fig.  5  is  shown  the  trip  lever  /  pivoted  at  J,  with  the  parts 
shown  in  Fig.  2  omitted  for  clarity.  The  electromagnet 
A  is  wound  with  many  turns  of  fine  insulated  copper  wire 
and  is  mounted  on  the  side  of  the  breaker  as  shown  in  Fig. 
13.  When  this  electromagnet  is  energized,  it  attracts  the  iron 
armature  B,  (pivoted  at  C)  and  this,  striking  a  stud  D  on  the 
tripping  lever,  causes  the  breaker  to  trip  or  open  as  before. 

The  shunt-trip  attachment  is  the  device  usually  employed 
when  protective  relays  are  used;  upon  the  occurrence  of  condi- 
tions for  which  it  is  set,  the  relay  closes  the  circuit  to  the  shunt- 
trip  attachment,  thereby  opening  the  breaker. 

Underload  Release. — There  are  cases,  not  important  enough 
to  warrant  the  use  of  an  accurate  relay,  where  a  circuit  must 
be  protected  from  damage  due  to  reverse  current  or  under 
current.  For  instance  if  the  charging  source  of  a  storage 
battery  is  interrupted,  the  battery  may  attempt  to  assume 
the  load,  or  to  motor  the  generator.  However,  the  instant 
the  current  drops  to  zero  (as  it  must  do  before  reversing),  the 
under-load  attachment  trips  the  breaker. 


CIRCUIT  BREAKERS  AND  RELEASES 


11 


The  principle  of  operation  is  shown  in  Fig.  6.  The  tripping 
lever  is  represented  by  J  and  7  as  before,  and  the  device  is  shown 
ready  to  apply  current.  An  electromagnet  N  (carrying  the 
main  current)  is  mounted  on  the  side  of  the  breaker  as  shown 
in  Fig.  13.  The  heavy  iron  armature  P  pivoted  at  C  has  on 
it's  side  a  small  lever  D  which  is  pivoted  at  E  and  is  manually 
hooked  over  the  stud  F,  thereby  holding  the  armature  a  slight 


FIG.  6. — Underload  attachment  (normal  position). 
FIG.  7. — Underload  attachment  (ready  to  trip). 
FIG.  8. — Underload  attachment  (tripped). 

distance  from  the  electromagnet  N.  When  the  electromagnet  is 
energized,  it  attracts  the  armature  P  to  the  position  shown  in 
Fig.  7,  thereby  releasing  the  catchlever  D. 

Now  should  the  current  fail,  the  armature  P  is  no  longer 
attracted  and  from  its  own  weight  falls  down  to  the  position 
shown  in  Fig.  8,  striking  the  trip  lever  a  sharp  blow  and  tripping 
the  breaker.  Care  must  be  taken  that  the  device  "sets" 
itself  when  current  is  applied;  that  is,  the  iron  weight  must 
be  attracted  and  the  lever  D  released,  otherwise  the  device 
will  not  operate  on  the  cessation  of  current. 


12 


PROTECTIVE  RELAYS 


Under-voltage  Release. — The  foregoing  device  when  wound 
with  many  turns  of  fine  wire  may  be  used  as  D.C.  under-voltage 
release,  operating  when  the  voltage  drops  to  a  predetermined 


FIG.     9. — Undervoltage  release  (normal  position). 
FIG.  10. — Undervoltage  release  (releasing). 
t  FIG.  11. — Undervoltage  release  (released) . 

amount.  On  alternating  current,  the  electromagnet  must 
have  a  closed-magnet  circuit,  and  so  the  armature  must  be  in 
the  same  position  (closed  air  gap)  in  both  operating  and  tripped 
position.  This  is  accomplished  by  a  toggle-joint  arrangement 


CIRCUIT  BREAKERS  AND  RELEASES  13 

as  shown  in  Fig.  9.  I  and  J  represent  the  trip  lever  as  before. 
The  coil  A  is  wound  on  laminated  iron  core  B  which  with  the  arma- 
ture C  forms  a  closed  magnetic  circuit.  Attached  to  the  back 
of  the  armature  is  a  triangular  piece  D  (pivoted  at  E)  which 
is  connected  to  the  setting  lever  F  (pivoted  at  G)  by  the  link 
H  which  forms  a  toggle  joint.  The  spring  K  tends  to  rotate 
F  in  a  clockwise  direction,  and  this,  by  attempting  to  straighten 
the  toggle,  would  tend  to  give  D  a  counterclockwise  rotation 
and  pull  the  armature  away  from  the  electromagnet.  This 
is  prevented  by  the  energizing  of  A.  As  soon  as  the  voltage 
drops  to  a  predetermined  amount,  the  armature  is  pulled  away 
allowing  the  springs  K  to  move  the  lever  F,  thereby  tripping 
the  lever  /  and  causing  the  breaker  to  open.  This  position 
is  shown  in  Fig.  10.  But  the  springs  pull  the  lever  F  further 
than  the  tripping  position  and  this  causes  the  link  H  and  piece 
D  to  force  the  armature  C  back  against  the  electromagnet, 
thereby  again  completing  the  magnetic  circuit  and  preventing  the 
coil  A  from  burning  out  should  the  voltage  come  on  again 
before  the  device  reset.  This  is  shown  in  Fig.  11.  Re- 
setting is  accomplished  simply  by  moving  the  knob  to  the  posi- 
tion in  Fig.  9.  A  breaker  with  this  form  of  release  is  shown 
in  Fig.  12. 

Over -voltage  Release. — For  an  over-voltage  release  a  device 
somewhat  similar  to  the  shunt-trip  attachment  is  used,  but  is 
usually  designed  with  a  heavier  magnetic  circuit  to  allow  a  smaller 
energy  loss  due  to  continuous  operation.  An  over-voltage 
release  is  shown  on  the  breaker  in  Fig.  14.  The  solenoid  is  wound 
on  a  heavy  iron-casting  and  has  in  its  center  an  iron  plunger. 
When  the  voltage  increases  to  a  certain  amount,  the  plunger 
is  sucked  upward,  striking  the  pivoted  lever  which  in  turn 
strikes  the  projection  on  the  trip  lever  and  trips  the  breaker. 

While  practically  every  make  of  breaker  varies  in  detail 
of  construction,  and  in  fact  each  manufacturer  may  turn  out 
various  designs  of  breakers,  yet  the  principle  of  operation 
is  practically  the  same  in  all  and  the  foregoing  examples  afe 
typical  of  the  devices  usually  employed. 

From  the  foregoing,  it  would  appear  as  if  the  protective 
relay  were  a  superfluous  device,  but  it  must  be  borne  in 
mind  that  these  releases  are  not  at  all  accurate  in  their  setting 


14 


PROTECTIVE  RELAYS 


FIG.    12. — Under  volt  age     release  FIG.    13. — Breaker    equipped    with 

mounted  on  breaker.  underload    release     and     shunt     trip 

attachment. 


FIG.  14. — Breaker  equipped  with  overvoltage  release. 


CIRCUIT  BREAKERS  AND  RELEASES  15 

and  may  not  always  give  adequate  protection.  On  high- 
tension  circuit  it  is  often  necessary  to  have  the  relays  where 
they  are  easily  accessible  as,  for  instance,  on  the  front  of  the 
switchboard,  while  the  breakers  may  be  in  some  remote  and 
practically  inaccessible  location.  Another  point  is  that  releases 
are  often  difficult  to  test  and  often  more  difficult  to  set  to  function 
on  the  desired  predetermined  conditions,  and  protective  relays 
are  becoming  more  and  more  to  be  recognized  as  a  necessary 
adjunct  to  the  well-equipped  power  plant  and  industrial 
concern. 


CHAPTER    III 

TRIP    CIRCUITS    AND    TIME    DELAYS 

Protective  relays  may  be  classified  as  circuit-closing  and 
circuit-opening  relays,  according  to  the  method  employed  to 
energize  the  trip  coil  of  the  circuit  breaker.  Circuit-closing 
relays  are  frequently  called  "shunt-trip"  relays.  It  has  already 
been  shown  how  the  circuit  breaker  may  be  opened  by  having 
an  electromagnet  which  actuates  the  trigger  when  it  is  energized. 
The  function  of  the  protective  relay  is  then,  to  complete  the 
electric  circuit  which  will  energize  this  electromagnet  or  trip 
coil  as  it  is  usually  called. 

To  accomplish  this,  the  relay  is  equipped  with  two  contacts; 
one  stationary  and  one  attached  to  the  moving  element.  Nor- 
mally these  are  not  touching,  but  when  the  abnormality  on  the 
circuit  reaches  a  certain  predetermined  magnitude,  the  moving 
element  of  the  relay  causes  the  contacts  to  touch  each  other 
or  to  close,  thus  completing  the  trip  circuit  and  energizing  the 
trip  coil  of  the  breaker,  and  causing  the  breaker  to  open. 

Trip-circuit  Sources. — It  is  evident  that  if  the  trip  coil  is 
wound  for  the  proper  voltage  and  frequency,  it  might  be  possible 
to  operate  the  trip  circuit  from  the  same  source  as  the  protected 
source.  But  with  no  protection  except  a  plain  protective  relay 
this  will  not  be  satisfactory,  as  in  the  event  of  a  heavy  short- 
circuit,  the  voltage  may  drop  so  low  that  there  is  not  enough 
voltage  to  operate  the  trip  coil,  even  though  the  relay  should 
close  the  trip  circuit;  and  it  will  be  noticed  that  this  low  voltage 
occurs  just  at  the  instant  when  full  voltage  is  most  needed. 

Even  the  practice  of  connecting  the  trip  circuit  to  an  exciter 
bus  of  the  D.C.  machines  supplying  the  field  of  large  alterna- 
tors is  not  satisfactory,  as  a  severe  disturbance  of  the  A.C. 
lines  may  be  felt  all  the  way  back  to  the  exciter  buses,  resulting 
in  a  failure  of  the  tripping  source  at  the  most  critical  moment. 

From  this  it  is  evident  that  the  tripping-circuit  source  must  be 
absolutely  dependable  and  absolutely  without  connection  to  the 

16 


TRIP  CIRCUITS  AND  TIME  DELAYS 


17 


source  being  protected.  The  most  dependable  and  suitable 
source  for  trip  circuits  is  therefore  a  good  reliable  storage  battery 
of  about  100  to  125  v.  It  should  constantly  be  borne  in  mind 
and  also  thoroughly  impressed  on  the  station  operators  that 
on  this  battery  depends  the  success  of  the  protective  system 
and  that  the  battery  must  always  be  in  good  condition  and 
always  in  service  as  there  is  no  telling  at  what  instant  a  heavy 
"short"  or  overload  will  occur,  and  if  the  battery  is  taken  out 
of  service  even  for  only  a  few  minutes,  yet  it  may  be  in  that  same 
interval  that  a  " short"  will  occur  and  burn  out  much  valuable 
apparatus.  A  diagram  of  connections  for  circuit-closing  tripping 
is  shown  in  Fig.  15. 


Current 
Transformer 


Circuif 
Breaker 


a 

•Wf^i 

-H- 

b 

=V» 

-— 

*'  Coil* 

fay 


To  Storage  Baffer) 


FIG.   15. — Elementary    diagram    of  a  circuit  closing  relay  with  separate  trip 

circuit. 

Auxiliary  Relays  or  Relay  Switches. — In  view  of  the  delicate 
operating  or  moving  elements  of  protective  relays,  and  their 
low  energy  consumption,  it  is  evident  that  they  cannot  have 
large,  bulky  contacts,  such  as  are  necessary  to  close  the  "heavy 
current  necessary  to  energize  the  trip  coils  of  the  larger  breakers. 
The  average  relay  is  designed  with  contacts  to  close  (but  never 
open,  however)  about  1  or  2  amp.  at  110  or  220  v.  When  the 
breaker-trip  coil  takes  more  than  this,  or  when  it  is  necessary 
to  operate  several  breakers  upon  the  functioning  of  one  relay, 
it  is  usual  to  have  the  protective  relay  close  the  circuit  to  an 
auxiliary  relay  (commonly  called  a  relay  switch,  or  a  multi- 
contact  relay),  and  this  auxiliary  relay  in  turn  closes  the  circuit 
which  energizes  the  trip  coil  of  the  breakers.  The  trip  circuit 
should  always  be  opened  by  auxiliary  contacts  on  the 
circuit  breaker  and  never  by  the  relay  contacts. 


18 


PROTECTIVE  RELAYS 


Auxiliary  relays,  with  their  diagrams  of  connections,  will  be 
fully  discussed  in  detail  in  another  chapter. 

Circuit-opening  or  Series  Trip. — There  are  many  installa- 
tions of  relays  for  plain  overload  protection  in  which  it  is  con- 
sidered too  expensive  to  install  and  maintain  a  complete  battery 
simply  for  tripping  purposes.  For  such  cases,  the  overload 
current  itself  is  used  as  the  energizing  source,  but  the  energiz- 
ing of  the  trip  coil  is  still  controlled  by  the  protective  relay. 
In  this  system,  the  trip  coil  is  put  in  series  with  the  load;  or 
in  parallel  with  a  shunt  which  is  in  series  with  the  load;  or 
connected  directly  to  the  secondary  of  a  series  transformer, 


.Circuit  Breaker 


FIG.   16. — Elementary  diagram  of  a  circuit  opening  relay. 

the  primary  of  which  carries  the  load  current.  But  the  protec- 
tive-relay contacts  are  normally  closed  and  are  connected 
so  that  they  short-circuit  the  trip  coil,  consequently  the  load 
current  (or  definite  fraction  of  it)  passes  through  the  low- 
resistance  contacts  and  very  little  flows  through  the  trip  coil. 
Now  should  an  unusual  disturbance  occur,  the  contacts  of  the 
protective  relay  will  open,  and  as  the  current  can  no  longer 
pass  through  the  contacts,  it  must  flow  through  the  trip  coil, 
thereby  energizing  it  and  tripping  the  breaker.  The  diagrams 
in  Fig.  16  show  the  connections  for  circuit  opening  or  series 
tripping. 

This  method  has  a  very  serious  drawback,  however;  if  the 
contacts  become  slightly  dirty,  or  make  poor  contacts  due 
to  vibration,  they  may  shunt  the  current  through  the  trip  coil 
under  normal  conditions  of  load  and  trip  the  breaker  without 


TRIP  CIRCUITS  AND  TIME  DELAYS 


19 


cause.  To  obviate  this  defect,  one  company  closes  the  contacts 
positively  by  means  of  a  toggle  joint,  and  this  gives  good 
protection  and  eliminates  the  danger  of  tripping  out  without 
cause. 

Transfer  Relay. — All  of  the  advantages  of  the  circuit-opening 
system  have  been  regained  and  the  former  defects  eliminated 
by  the  development  of  the 
so-called  ' '  transfer  relay ' ' 
which  is  shown  in  Fig.  17.  I 
While  this  relay  is  discussed 
at  length  in  another  chapter, 
yet  it  may  be  here  stated 
that  this  system  uses  a  cir- 
cuit-closing relay  as  the  pro- 
tective device  and  when  this 
relay  functions,  it  short- 
circuits  a  "holding-down" 
coil  on  the  transfer  relay; 
this  allows  the  transfer  relay 
to  function  and  in  doing  so 
it  breaks  the  series  circuit 
and  instantly  cuts  the  trip 
coil  into  this  circuit,  thereby 
tripping  the  breaker. 

Time  Delays.— -  Were  it  not 
for  the  fact  that  it  is  neces- 
sary to  have  a  certain  time 
delay  between  the  instant  of 
disturbance  and  the  instant 
of  breaker  functioning,  there 
would  be  but  little  field  for 
the  protective  relay.  There  | 
are  three  definite  classes  into 

,  .   ,         ,  .        ..    .  ,     ,      FIG.  17. — Westmgnouse  transfer    relay. 

which  relays  may  be  divided 

according    to    time     delay:    instantaneous,    inverse-time    and 

definite-time. 

As  the  name  implies,  the  instantaneous  relay  provides  no 
time  delay  between  the  instants  of  disturbance  and  tripping. 
Instantaneous  relays  are  used  generally  where  accurate  protec- 


20 


PROTECTIVE  RELAYS 


tion  is  desired,  and  where  there  is  almost  no  possibility  of  the 
disturbance  clearing  itself  in  a  few  seconds.  For  instance, 
if  a  transformer  or  a  generator  develops  an  internal  short- 
circuit,  there  is  practically  no  chance  of  it  clearing  itself  and  the 

transformer  or  generator 
should  be  cut  out  instantly. 
Or  if  a  generator  loses  its  field 
or  if  a  battery  which  is  sup- 
posed to  be  only  charging, 
should  suddenly  start  dis- 

Fusing    Current    in    Amperes  charging     due     to     the    failure 

FIG.  18. — Characteristic  time-load  curve     of  the   charging    source,   it    is 

of  a  6-ampere  fuse.  advisable  for  the   circuit  to 

be  opened  instantly,  and  for  this  purpose,  the  instantaneous 
protective  relay  is  employed. 

Inverse  Time  Limit. — This  kind  of  time  limit  was  the 
favorite  for  many  years  as  it  gave  protection  commen- 
surate with  the  magnitude  of  the  overload.  By  examining 
the  time-delay  curve  of  an  ordinary  fuse  (Fig.  18)  it  will  be 


Time  in  Seconds 

_O  fO  4*  0  »  c 

\ 

\ 

\ 

\ 

1  —  . 

—    — 

• 

•         • 

)                5                10               15               20             2£ 

40 


50 


60. 


70  $0  90        100 

Garment  in  Amperes 

FIG.  19. — Characteristic  time-load  curves  of  G.  E.  inverse  time  limit,  bellows 
type,  overload  relay. 

seen  that  this  inverse  time  protection  is  given,  but  of  course 
on  very  heavy  overloads  the  time  becomes  almost  instantaneous. 
Figure  19  gives  a  typical  curve  of  a  plain  inverse-time-limit 


TRIP  CIRCUITS  AND  TIME  DELAYS 


21 


relay  and  readily  shows  how  the  time  varies  with  the  extent 
of  the  overload.  For  instance,  consider  the  top  curve.  This 
is  taken  with  the  relay  set  to  close  the  contacts  at  10  amp.  at 
which  setting  it  takes  about  10  sec.,  this  being  on  an  indefinite 
part  of  the  curve. 

With  the  same  setting  and  20  amp.  applied  (i.e  200  per  cent 
of  load  or  100  per  cent  overload)  it  takes  only  2.5  sec.;  at  50  amp. 
(i.e.  500  per  cent  of  load  or  400  per  cent  overload)  it  takes 
1.3  sec.;  at  100 amp.  it  takes  only  0.4  sec.,  and  above  this  the 
relay  is  practically  instantaneous. 

Definite -time  Delays. — As  the  name  implies,  in  this  type 
of  protective  relay  there  is  a  definite  time  delay  between  the 


t& 

C* 


/n 

ON 


SQ 


100 


JO  ZO  SQ  40  SO  60  7< 

Current  in  Amperes 

FIG.  20. — Characteristic  time-load  curves  of  a  G.  E.  definite  time  limit,  bellows 
type,  overload  relay. 

instant  of  disturbance  and  the  closing  of  contacts  and  this 
time  delay  is  in  no  way  affected  by  the  magnitude  of  the  ab- 
normality. Such  time  delays  have  been  successfully  employed 
on  a  small  scale,  but  have  rapidly  given  way  to  the  time  delay 
described  in  the  next  paragraph.  Typical  definite  time  curves 
are  shown  in  Fig.  20. 

Inverse-definite-time  Delays. — In  this  type  of  delay  the  latest 
practice  is  obtained  by  having  the  protective  relay  give  a 
time  delay  which  is  inversely  proportional  to  the  magnitude 
of  the  overload  up  to  about  1,000  per  cent  of  load,  but  which 


22 


PROTECTIVE  RELAYS 


becomes  a  definite  time  limit  upon  any  greater  overload  than 
this.  For  instance,  consider  the  curve  in  Fig.  21,  which  is 
the  time-delay  curve  of  a  modern  induction-type  overload 


«* 

t 

5s 

c  3 

«  2 
£ 

£   ' 
I 

^ 

^S, 

•*-= 

===; 

~~" 

— 

•^H 

00 

500                         1000                        1500                      200 

Per  Cent,   of   Amperes    Necessary  to  Close  Contacts 

Fiq.  21. — Typical  time-load  curve  of  a  Westinghouse  induction  type  overload 

relay. 

protective  relay.  At  200  per  cent  of  load  the  time  delay  is 
5  sec.;  at  300  per  cent,  3.5  sec.;  at  500  per  cent,  2.5  sec.;  and  at 
1,000  per  cent  and  any  overload  above  1,000  per  cent,  the  time 
delay  is  a  definite  2  sec.  Figure  22  shows  another  set  of  curves 
where  definite  time  is  approached  at  very  heavy  overloads. 


/O  45  20  25  30  35  49  45  50 

Time*  Minimum  Operating  Current 

Fio.  22. — Typical  time-load  curves  of  a  G.  E.  induction  type  overload  relay. 

The  applications  of  these  time  delays,  and  how  to  set  the 
various  relays  to  obtain  certain  delays  will  be  treated  in  detail 
in  later  chapters. 


TRIP  CIRCUITS  AND  TIME  DELAYS  23 

How  Time  Delays  Are  Obtained. — The  method  of  lagging 
or  damping  the  moving  element  of  a  protective  relay  depends 
largely  on  the  principle  of  operation  of  the  relay.  In  the  direct- 
current  type  employing  a  moving  coil  and  permanent  or  electro- 
magnet, the  time  delay  is  obtained  by  the  use  of  an  aluminum 
or  copper  bobbin  which  also  serves  as  a  support  for  the  winding. 
It  takes  power  to  move  the  bobbin  through  the  intense  field 
and  thus  the  movement,  and  consequently  the  time  delay, 
is  inversely  proportional  to  the  power  applied,  or  in  other  words, 
to  the  overload. 

In  the  solenoid  and  plunger  type,  some  manufacturers  employ 
a  leather  bellows  with  a  small  adjustable  needle  valve  to  allow 
the  air  to  escape  slowly.  As  the  plunger  attempts  to  rise, 
the  air  is  compressed  in  the  bellows,  thus  retarding  the  movement. 
Other  manufacturers  use  a  dashpot  with  oil  to  retard  the  motion. 

In  the  induction  type,  an  aluminum  disk  rotates  between 
strong  permanent  magnets  which  retard  the  motion.  In 
this  type,  the  definite  time  is  obtained  by  having  a  small  trans- 
former which  saturates  on  heavy  overload,  thus  limiting  the 
power  which  is  supplied  to  the  relay  windings. 

Other  types  use  various  novel  methods  which  will  be  fully 
described  under  the  various  types  of  protective  relays. 


CHAPTER  IV 

PLUNGER-TYPE  PROTECTIVE  RELAYS 

A  relay  operating  on  the  effect  of  a  solenoid  to  raise  an  iron 
plunger,  thus  closing  or  opening  contacts,  is  shown  in  Fig.  23. 
Referring  to  the  diagram  of  parts  shown  in  Fig.  24,  winding 
A  is  wound  around  the  iron  core  B.  Supported  at  the  two  poles 
N  and  S  is  an  iron  plunger  C  arranged  so  that  it  may  slide  up 


A< 


FIG.    23. — Condi  t     strap     wound, 
plunger  type  overload  relay. 


FIG.  24. — Schematic  diagram  of  relay 
shown  in  Fig.  23. 


and  down.     When  the  current  in  A  reaches  a  certain   value, 
the  iron  core  C  is  lifted,  thus  closing  thejeontajrts __Z)__and_/? 
f  withL^brjdge^F,   which  will  immediately  trip  the  breaker,  as 
previously  described. 

When  the  current  is  greater  than  1,000  amp.,  a  winding  is  not 
necessary,  as  the  magnetism  from  the  straight  bar  or  cable 
produces  sufficient  flux  to  operate  the  relay.  The  relay  may 
then  take  the  form  shown  in  Fig.  25,  the  cable  simply  passing 
through  the  large  hole  H,  which  is  surrounded  by  insulating 
material.  In  Fig.  26  is  another  modification  which  may  be 
used  on  a  busbar  that  runs  vertically  instead  of  horizontally. 

24 


PLUNGER-TYPE  PROTECTIVE  RELAYS 


25 


In  this  case  the  magnetic  circuit  is  simply  clamped  around  the 
busbar,  which  is  of  course  insulated. 


FIG.  25. — Overload  relay  arranged  for       FIG.  26. — Overload  relay  arranged  for 
horizontal  bus.  vertical  bus. 

Adjusting  Relays. — In  order  to  adjust  these  relays  to  operate 
on  various  loads,  the  plunger  C,  Fig.  23,  is  arranged  with  a  nut 


FIG.  27. — G.  E.  strap  wound  overload       FIG.  28. — Schematic  diagram  of  relay 
relay.  shown  in  Fig.  27. 

G  by  means  of  which  the  plunger  may  be  raised  or  lowered  on 
the  stem  H.     Thus  if  the  plunger  is  at  the  lowest  point,  it  will 


26 


PROTECTIVE  RELAYS 


takeji  maximum  jof_  cur  rent  to  raise  it,  but  if  it  is  set  high,  then 
it  will  rise  on  a  minimum  of  current. 

Another  form  of  overload  (excess-current)  relay,  utilizing  the 
same  principle  of  operation  as  those  already  described,  is  shown 
in  Fig.  28.  Coil  A  is  wound  on  the  central  part  over  the  iron 
plunger  C,  and  the  magnetic  circuit  is  completed  by  the  two 
parts  B  and  BI.  The  action  is  identical  with  the  previously 
described  relay;  namely,  when  the  current  reaches  a  certain 
value,  the  plunger  C  is  lifted  upward,  thus  causing  the  contact 


FIG.  29.  FIG.  30. 

FIG.  29. — Interior  view  of  obsolete  Westinghouse  definite  time  limit  D.C. 
relay. 

FIG.  30. — Shows  the  cover  on  and  the  time  setting  arm. 

disk  F  to  short-circuit  the  two  contacts  D  and  E,  which  complete 
the  circuit  that  trips  out  the  breaker.  In  another  type  of 
small  capacity,  adjustment  is  made  by  using  taps  on  the  wind- 
ing; however,  this  cannot  be  done  in  capacities  of  several  hundred 
amperes.  The  great  advantage  gained  by  the  simple  relay  de- 
scribed further  on  has  discouraged  the  use  of  plunger-type  relays 
on  direct-current  circuits.  If  a  plunger-type  relay  is  to  be 
used  with  a  shunt,  as  has  been  done  in  rare  cases,  the  adjust- 
ment for  load  is  made  by  varying  the  drop  of  the  shunt. 


PLUNGER-TYPE  PROTECTIVE  RELAYS 


27 


Figure  29  shows  a  definite-time  relay  with  the  cover  removed 
and  Fig.  31  gives  a  schematic  diagram  of  parts.  The  solenoid 
A  has  an  iron  plunger  B  which  under  normal  condition  rests 
on  the  moving  arm  C,  pivoted  at  F,  which  carries  a  contact 
D  and  a  counterweight  E.  When  the  solenoid  A  is  energized, 
the  core  B  is  raised  upward  instantly;  relieved  of  this  weight, 
the  counterweight  E  now  causes  the  contact  D  to  start  upward 
to  meet  the  upper  contact  G.  However,  attached  to  the  arm 
C  is  a  piston  H  working  within  a  cylinder  7,  which  retards  the 
movement  of  arm  C,  making  it  move  very  slowly  as  the  air 
escapes  around  the  plunger.  Then,  after  a  definite  time,  from 
1  to  5  sec.,  depending  on  the  initial  distance  between  contacts 
D  and  G,  the  contacts  D  and  G  close,  thus  closing  the  circuit  to 
the  shunt-trip  coil  of  the  circuit  breaker,  causing  the  latter  to 
open. 


Fio.  31. — Schematic  diagram  of  the  definite  time  limit  overload  relay,  Fig.  30. 


If  the  current  drops  to  normal  before  contacts  G  and  D, 
Fig.  31,  close,  the  solenoid  allows  the  plunger. B  to  drop,  thus 
forcing  the  arm  C  downward  into  normal  position.  In  order 
that  the  relay  may  reset  quickly,  a  valve  is  provided  in  the 
dashpot  plunger.  This  valve  consists  of  a  little  steel  ball  J, 
which  closes  the  air  ports  K  when  the  piston  moves  upward  and 
attempts  to  force  air  out  of  the  port,  but  raises  and  allows  the  air 
to  enter  readily  when  the  piston  moves  down  as  in  resetting. 
Figure  30  is  an  outside  view  of  a  definite-time-limit  relay  similar 
to  that  in  Fig.  29.  This  relay  will  close  the  circuit  in  the  number 
of  seconds  that  arm  A  points  to  on  scale  S.  This  relay  is  now 
practically  obsolete,  but  there  are  still  many  in  old  installations. 


28 


PROTECTIVE  RELAYS 


The  types  of  relays,  Figs.  23  to  28,  if  desired,  may  close  the 
circuit  to  a  definite-time-limit  relay  instead  of  tripping  the 
breaker  instantly,  but  then,  while  the  action  is  selective  the  cost 
renders  its  use  prohibitive.  Selective  action,  except  in  very 
heavy  short-circuits,  may  be  obtained  by  lagging  the  time  of  the 
tripping,  and  making  the  relay  an  inverse-time-limit  device. 


FIG.  32. — View  of    G.  E.  series  type 
overload  inverse-time-limit  relay. 


FIG.  33. — G.    E.    unit   type     bellows 
type  overload  relay. 


That  is,  the  greater  the  overload  the  quicker  the  time.  In 
fact,  in  actual  practice,  the  instantaneous  relay  has  a  very  limited 
use;  an  inverse-time-limit  relay  costs  only  slightly  more,  gives 
the  same  protection  and  will  not  interrupt  service  on  transient 
short-circuits.  The  latter  type  is  shown  in  Figs.  32  and  33. 
The  plunger,  in  rising,  compresses  the  air  in  the  leather  bellows 


PLUNGER-TYPE  PROTECTIVE  RELAYS 


29 


B,  which  resists  its  upward  movement.  In  the  top  of  the  cast- 
ing to  which  the  bellows  is  attached,  is  an  air  passage  C,  which 
may  be  anywhere  from  1  or  2  sec.  to  20  or  30  sec. 

The  greatest  objection  to  the  bellows-type  relay  is  that  the 
leather,  unless  carefully  attended  to,  will  dry  out  and  crack, 
making  the  permanence  of  time  setting  very  unreliable.  To 
secure  the  best  operation  the  bellows  should  be  rubbed  with 
neatsfoot  oil  every  few  months,  and  load-time  curves  taken. 
Otherwise  the  relays  may  fail  at  a  critical  time.  Another  fault 
is  that,  while  the  time  is  inverse  up  to  certain 
overloads,  on  short-circuits  the  time  is  almost 
instantaneous.  Therefore,  if  applied  to  a 
radial-feeder  system,  the  action  will  be  select- 
ive up  to  certain  overload,  but  above  this  a 
breaker  near  the  generator  may  go  out  as 
quickly  as  a  breaker  near  the  source  of  dis- 
turbance. To  overcome  this  difficulty,  a 
plunger-type,  overload,  definite-time-limit 
relay  was  devised. 

Time  -limit  Relays.  —  A  definite-time-limit 
relay  is  shown  diagrammatically  in  Fig.  34. 
The  plunger  A  is  not  rigidly  attached  to 
the  stem  B,  as  in  the  type  previously  de- 
scribed, but  slides  freely  on  it.  If  an  overload 
occurs  the  plunger  is  raised  and  compresses 
the  spring  C  which  in  turn  forces  the  stem 
B  upward  against  the  resistance  of  the  bellows 
D  and  finally  closes  the  contacts  E  and  F  circuit^ciosmg, 
with  the  disk  G.  It  will  be  seen  readily  that 
no  matter  how  severe  the  overload  may  be, 
it  can  only  compress  the  spring  C;  consequently,  the  upward 
pressure  on  the  bellows  stem  is  constant  regardless  of  overload 
and  the  time  is  therefore  constant.  The  duration  of  time  is 
varied  by  opening  or  closing  the  air  valve  S  as  described  for 
inverse-time-limit  types.  At  the  first  glance  this  might  appear 
the  solution  of  radial  protection,  but  it  is  impossible  to  depend 
on  the  relay  for  closer  settings  than  1  sec.;  therefore,  when 
there  are  four  or  five  relays  connected  in  a  circuit,  those  near 
the  generators  must  be  set  to  operate  in  about  5  or  6  sec.,  which 


FIG.  34.—  Schematic 


defi- 


30 


PROTECTIVE  RELAYS 


is  too  long  a  time  to  sustain  a  dead  short-circuit,  especially 
near  the  generator.  Then,  too,  the  relays  would  trip  just  as 
quickly  on  a  moderate  overload  as  on  a  heavy  overload,  which  is 
not  at  all  desirable.  Were  the  foregoing  of  great  importance 
it  would  be  necessary  to  perfect  a  relay  accurate  within  small 
percentage  of  sustained  accuracy  and  one  whose  curve  was  inverse 
up  to  certain  overloads,  after  which  it  would  become 
a  definite-time-limit  device.  However,  owing  to  the  unques- 
tioned superiority  of  alternating  current  for  high-tension 
long-distance  transmissions  and  the  comparatively  small  size  of 
most  direct-current  radial  systems  of  transmission,  relay  engi- 


1111 


il  I 


FIG.  35. — Elementary  diagram  of  radial  distribution  system. 

neers  have  devoted  most  of  their  energies  to  the  perfection  of  alter- 
nating-current relays  which  are  to  a  high  degree  perfect  in  their 
protection.  In  the  large  power  plants  or  factories,  however, 
where  there  are  numerous  machines  that  must  be  kept  running 
unless  actually  damaged,  a  radial  system  of  protection  may 
be  adopted  with  success. 

This  brings  up  an  important  use  for  definite-time-limit 
relays.  Consider  the  distribution  system  shown  in  Fig.  35. 
Each  time  the  line  divides  to  supply  a  set  of  feeders,  a  definite- 
time-limit  relay  is  supplied  to  operate  a  double-pole  circuit- 
breaker.  For  instance,  the  feeder  from  the  busbar  is  protected 
by  breaker  A,  the  next  subdivisions  are  protected  by  breakers 
B  and  C,  and  the  next  by  circuit  breakers  D,  E,  F  and  G.  Sup- 
pose a  heavy  overload  occurs  on  the  feeder  protected  by  breaker 


PLUNGER-TYPE  PROTECTIVE  RELAYS 


31 


D.  The  excess  current  extends  all  the  way  back  to  the  main 
bus,  and  were  definite-time  relays  not  used,  breaker  A  would 
go  out  as  soon  as  breaker  Z>,  thus  interrupting  every  circuit 
connected  to  the  feeder  protected  by  breaker  A.  But  this 
is  where  the  definite-time-limit  relay  enters  in.  The  relay 
at  D  is  set,  say,  for  1  sec.,  B  and  C  for  2  sec.,  and  A  for  3  sec. 


FIG.  36. — View  of  Westinghouse  bellows  type  overload  relay  with  and  without 

protecting  cover. 

Thus  when  the  disturbance  occurs,  all  the  relays  of  breakers  A, 
C  and  D  start  to  operate,  but  at  the  end  of  1  sec.,  breaker  D 
opens,  relieving  the  excess  current,  and  all  the  other  relays 
reset  quickly,  confining  the  disturbance  to  the  one  line  on  which 
it  occurred.  Had  the  disturbance  occurred  on  feeder  C,  then 
the  breaker  at  C  would  have  gone  out  in  2  sec.;  breaker  A 
would  not  have  had  time  to  open  and  feeder  B  would  not  have 
been  interrupted.  Figure  36  shows  a  typical  overload  relay 
which  obtains  the  time  limit  by  means  of  an  air  bellows,  and  Fig. 


32 


PROTECTIVE  RELAYS 


37  is  a  schematic  diagram  of  the  same.  The  iron  plunger 
A,  working  under  the  influence  of  the  solenoid  B,  carries  at 
its  lower  end  an  insulated  disk  D  having  on  its  circumference 
a  band  of  nonoxidizing  metal.  When  the  solenoid  coil  B  is 
energized  by  a  current  of  a  certain  strength,  the  core  A  is  pulled 
upward,  thus  forcing  the  disk  D  against  the  contacts  E  and 
F,  completing  a  circuit  to  a  shunt-trip  coil  on  a  circuit  breaker. 
On  the  upper  end  of  the  plunger  shaft  is  a  leather  bellows  G 
fastened  to  the  permanent  support  H,  so  that  as  the  plunger 
rises,  it  compresses  the  air  in  the  bellows  and  resists  its  upward 

motion.  This  air  is  permitted  to  escape 
gradually  through  a  little  opening  at  7 
in  the  casting,  and  this  gradual  escape 
of  air  allows  the  plunger  to  rise  in  a 
certain  time  to  complete  contact  at  E 

SLTidF. 

It  is  evident  that  the  greater  the 
current  in  the  solenoid  B  the  greater 
will  be  the  upward  pull,  consequently 
the  greater  the  compression  in  the  bel- 
lows, the  quicker  the  escape  of  air  and 
the  shorter  the  time.  That  is,  the 
greater  the  overload  the  quicker  the 
FIG.  37. — Schematic  dia-  circuit  will  be  opened. 

Sh°Wn   ln       This  inverse  proportion  is  not  a  fixed 
factor,   however,  since  by  varying  the 

size  of  the  escape  aperture  at  I  by  means  of  the  valve  J  the 
time  may  be  varied  from  almost  nothing  to  15  or  20  sec.,  and 
still  have  the  inverse  time  limit. 

The  taps  K,  L,  M,  etc.,  are  provided  to  change  the  number 
of  turns  in  circuit  and  consequently  the  amount  of  current 
required  to  operate  the  relay.  For  instance,  this  particular 
relay  has  taps  for  4,  5,  6,  7  and  8  amp.  This  means  that  if  the 
4-amp.  tap  is  in  circuit,  the  plunger  wiU  start  to  rise  when  the 
current  reaches  4  amp.;  with  the  5-amp.  tap,  the  plunger  will 
rise  when  the  current  reaches  5  amp.,  etc. 

As  a  practical  example,  assume  that  the  relay  is  used  with  a 
100  to  5  current  transformer.  This  means  that  when  the 
full-load  current  of  100  amp.  is  reached  there  will  be  5  amp. 


PLUNGER-TYPE  PROTECTIVE  RELAYS 


33 


passing  through  the  relay  coil.  But  if  connections  are  made 
with  the  4-amp.  tap,  the  relay  will  operate  on  4  amp.,  which, 
in  the  ratio  of  5:100,  means  that  it  requires  only  80  amp.  to 
operate  the  relay  with  this  setting.  Should  the  8-amp.  tap  be 
connected  in,  the  relay  will  not  operate  until  there  is  a  load 
of  160  amp.  on  the  primary  of  the  transformer.  This  might 
take  18  sec.  to  trip  the  oil  switch.  But  with  the  same  setting, 
a  load  of  300  amp.  might  take  only  4  sec.;  600  amp.  1.5  sec.; 
1,000  amp.  1  sec.;  and  so  on  in  time  inversely  proportional  to 
the  current. 


200 


IWO     1600 


400        WO       600       1000       1200      1400 
Per  Cent  Load  Required  to  Trip 

FIG.  38. — Characteristic  time  load  curves  of  Westinghouse  inverse  time  limit, 
bellows  type  overload  relay. 

This  inverse  time  is  not  a  straight-line  inverse  time,  but 
follows  a  curve  such  as  that  shown  in  Fig.  38.  It  will  be  noticed 
that  this  curve  is  very  similar  to  the  time  curve  of  a  fuse,  but 
has  the  distinct  advantage  that  it  may  be  varied  at  will,  and 
the  relay  is  accurate  and  can  be  depended  upon,  and  is  auto- 
matically reset  each  time  it  operates. 

One  relay  is  necessary  for  the  protection  of  a  single-phase 
circuit  and  two  for  a  two-phase  or  a  three-phase  circuit,  although 
three  on  three-phase  give  better  protection.  Figure  39  shows 
the  diagram  of  connections  of  a  single-phase  circuit.  A  is  the 
busbar  supplying  the  feeder  B,  which  in  turn  supplies  the  load 


34 


PROTECTIVE  RELAYS 


C,  and  the  circuit  breaker  D  opens  and  closes  the  circuit.  E 
is  the  primary  winding  of  a  series  transformer  and  F  the  second- 
ary, which  is  connected  to  the  relay  and  energizes  the  solenoid 
H.  The  direct-current  circuit  is  connected  to  the  shunt-trip 


F 

TRIP  COIL 

**• 

TO  DIRECT-CURRENT  TRIPPING  CIRCUIT 

G 

FIG.  39. — Elementary  diagrams  of  circuit  closing  relay  on  single  phase  circuit. 


SCRIfS  TRANSFORMERS 


TO  DIRECT-CURRENT 
TRIPPING  CIRCUIT 


SINGLEPHASl 


TWO  PHASE 


THREEPHASE 

FIG.  40. — Standard   diagrams  of   connections  of   Westinghouse   bellows    type, 
circuit  closing,  overload  relays. 

coil  of  breaker  D,  with  a  break  in  the  circuit  at  contacts  J 
and  K.    This  diagram  shows  the  normal  operating  position. 
If  an  overload  occurs  at  C  and  excess  current  in  the  trans- 
former EF  results,  the  relay  plunger  rises,  short-circuits  the 


PLUNGER-TYPE  PROTECTIVE  RELAYS 


35 


contacts  JK  and  completes  the  trip  circuit;  an  instant  later 
the   oil   switch   will   open,   disconnecting   the  line.     Figure  40 


Cover 

Needle  Valve  Adjusting  Nut 
Lock  Nut 

Bellows  Support 

Tap  for  Quick  Release  Valve 
When  Used 


Bellows 

Compression  Spring 
Stationary  Contact 

Moving  Contact 

Contact  Base 
Plunger  Stop 

Frame 

Magnet  Frame  Cover 

Pole  Piece 

Plunger 
Operating  Coil 

Magnet  Frame  Shell 

Magnet  Frame  End  Piece 

Magnet  Frame  Supporting 

Screw 

Calibrating  Rod 
Calibrating  Tube 

Dust  Cover 


Adjusting  Nut 


FIG.  41. — G.  E.  unit  plunger  type  relay. 

gives  diagrams  of  relay  and  transformer  connections  for  pro- 
tecting single-phase,  two-phase  and  three-phase  circuits. 


36 


PROTECTIVE  RELAYS 


Other  Types. — Another  bellows  type  of  relay  is  shown  in 
Fig.  41.  This  is  a  single-phase  unit.  Many  relays  were  formerly 
made  with  two  and  three  relays  mounted  on  one  casting  to  pro- 


Air  Valve  for  Time 
Adjustment 


Quick  Return 
Valve 

I 


FIG.  42. — Air-valve  and  quick  resetting  valve  on  G.  E.  bellows  type    relay. 

tect  two-  or  three-phase  circuits.  These  will  still  be  found 
on  older  installations.  However,  they  require  two  or  three  sepa- 
rate series  transformers  just  as  do  the  relays  previously  described. 


PLUNGER-TYPE  PROTECTIVE  RELAYS 


37 


It  will  be  noted  in  this  type  that  the  contacts  are  at  the  top 
and  protected  by  a  removable  cover,  permitting  ready  inspection. 
In  this  relay  the  various  load  settings  are  not  obtained  by 
taps,  but  by  varying  the  position  of  the  iron  plunger  in  the 
solenoid  by  an  adjustment  at  A.  Lowering  the  plunger  re- 
quires more  current  to  raise  it,  and  raising  the  position  requires 
less  current. 


FIG.  43. — Early  form   of  quick  resetting  air  valves  on  bellows  type  relay. 


The  time  setting  is  varied  by  an  air  valve  in  the  top  of  the 
bellows  casting,  as  shown  in  Fig.  42.  When  an  overload  occurs, 
the  solenoid  raises  the  plunger,  thereby  raising  the  stem  A. 
This  compresses  air  in  the  leather  bellows  B,  forcing  it 
up  the  channel  C,  and  through  the  valve  D.  The  time 
is  adjusted  by  varying  the  position  of  the  pin  C  by  turning 
G.  A  good  feature  is  the  quick-resetting  device  shown  at 
H  and  K.  K  is  a  cylinder  which,  on  upward  travel  of  the  stem 
A,  is  forced  against  opening  L  and  effectually  closes  opening 
M .  When  the  relay  attempts  to  reset,  the  air  is  rarefied  in  the 
bellows  and  cylinder  K  is  lifted,  allowing  air  to  enter  M  and 
P,  thereby  insuring  a  quick  resetting  of  the  plunger. 


38 


PROTECTIVE  RELA YS 


An  early  form  of  quick  resetting  device  is  shown  in  Fig.  43,  but 
one  of  the  greatest  drawbacks  of  this  type  is  that  on  a  heavy 
alternating-current  overload,  especially  low  frequency,  the  peak 
of  the  wave  causes  the  air  to  compress  in  the  bellows  while  when 
the  A.C.  wave  passes  through  zero,  this  compressed  air  drives 


FIG.  44. — G.   E.   dashpot  type,  circuit-opening,  inverse-time-limit  relay,  with 
and  without  protecting  cover. 

the  plunger  down  a  trifle  and  then  the  quick-resetting  device 
admits  air  the  wrong  time.  This  recurring  at  every  alterna- 
tion drove  the  plunger  so  far  down  that  it  could  not  close  the 
contacts. 

One  drawback  to  the  leather-bellows-type  relay  is  that  the 
leather,  unless  attended  to  carefully,  will  harden  in  time  and 
crack,  thus  defeating  the  purpose  of  an  accurate  time.  To 
overcome  this,  relays  of  the  type  shown  in  Figs.  44  were 


PLUNGER-TYPE  PROTECTIVE  RELAYS 


39 


developed.  These  had  the  contacts  at  the  top,  as  in  the  type 
Fig.  45,  but  the  bellows  is  omitted  and  an  oil  dashpot  was  placed 
at  the  bottom  of  the  stem  as  shown  in  Fig.  46. 

Instead  of  forcing  air  through  a  needle  valve,  oil  is  forced 
by  a  piston  on  its  upward  travel  through  the  valve  E  and  out 


FIG.  45.  FIG.  46. 

FIG.    45.  (Left) — General   Electric   bellows   type  inverse-time-limit,    circuit- 
closing  relay. 

FIG.  46.  (Right) — Shows  details  of  the  oil  valve  in  the  oil-damped  relays. 

of  hole  F,  Fig.  46.  The  piston  C  has  a  number  of  holes  in  the 
bottom,  which  are  normally  covered  by  the  disk  D.  Upon  upward 
travel  the  disk  closes  the  holes  practically  oiltight,  but  on 
downward  travel  it  rises  and  allows  a  quick  resetting  of  the 
plunger. 


40 


PROTECTIVE  RELAYS 


These  relays  cannot  be  used  where  they  are  subjected  to  extreme 
changes  in  temperature,  and  no  other  oil  except  that  supplied 
by  the  manufacturers  should  be  used  in  the  dashpot.  Their 
time  may  be  varied  from  almost  instantaneous  at  heavy  loads, 
to  over  5  min.  at  150  per  cent  load. 

Sometimes,  as  in  radial  systems,  an  inverse  time  is  not  so 
desirable  as  a  definite  tune.  To  obtain  this,  the  relays  hereto- 
fore shown  are  slightly  modified  so  that  instead  of  the  plunger 
being  rigidly  attached  to  the  bellows  and  contact  shaft,  the  rais- 
ing of  the  plunger  merely  compresses  a  spring,  which  in  turn 


FIG.  47. — Elementary  diagram  of  connections   of  series  type   circuit  opening 

relay. 

raises  the  shaft.  This  was  shown  in  Fig.  34.  The  plunger  A 
is  free  to  slide  up  on  the  shaft  B.  When  it  rises,  it  compresses 
the  spring  C,  which  presses  against  a  stop  rigidly  attached  to 
the  shaft,  at  the  bottom  of  the  bellows,  thereby  raising  the 
shaft  and  contacts. 

It  will  be  readily  seen  that  no  matter  how  much  current 
is  passed  through  the  solenoid,  once  it  rises,  there  can  be  no 
greater  compression  from  the  spring  no  matter  what  the  load. 
Consequently,  the  contacts  are  closed  in  a  definite  time,  depending 
upon  the  time  setting  after  the  raising  of  the  plunger.  Hence 
the  name,  "definite-time-limit  overload  relay." 

In  the  examples  shown,  it  was  assumed  that  there  was  always 
a  separate  direct-current  circuit  which  the  relay  completed 
to  trip  the  oil  switch.  Sometimes  a  source  of  direct  current 
is  not  available,  so  the  secondary  current  of  the  series  trans- 


PLUNGER-TYPE  PROTECTIVE  RELAYS 


41 


Phase  A-^ ..       Phase  B 


Switch. 


/                                Wr* 
(If  VS( 

A 

^  Series 
<^    Trnnsf 

TFF 

ipsJ 

§ 

v" 

Tr/p 
Coils 

Rela^ 

/ 

?«>/<*/ 

1  1 

Single  Phase  2  Phase  3  Phase 

FIG.  48. — Standard  diagrams  of  connections  for  Westinghouse   bellows  type 
overload  circuit  opening  relays. 


CONTACTS 


Single  Phase 


SOURCE 


Three  Phase  Ungrounded 


Three  Phase  Grounded  Neutral  Quarter  Phase 

FIG.  49. — Standard  diagrams  of  plunger  type  circuit  closing  overload  relays  as 
recommended  by  the  General  Electric  Co. 


42 


PROTECTIVE  RELAYS 


former  is  used  to  trip  the  circuit  breaker.  These  relays  are 
known  as  series-trip  or  circuit-opening  types.  The  secondary 
of  the  transformer  is  connected  to  the  trip  coil  on  the  breaker, 
but  this  coil  is  short-circuited  normally  by  the  relay  contacts. 
When  the  relay  operates,  it  connects  the  trip  coil  in  circuit, 
thereby  causing  all  the  current  to  flow  through  it  and  open 


RELAY 


Single  Phase 


^  GROUND 
*£-A/ww3i 

Three  Phase  Ungrounded 


OIL  CIRCUIT  BREAKER 


Three  Phase  Grounded  Neutral  Quarter  Phase 

Fia.  50. — Diagrams  corresponding  to  Fig.  49  except  for  circuit  opening  relays. 

the  oil  switch.  This  is  shown  in  Fig.  47,  in  which  the  feeder  E 
is  supplying  the  load  C  through  the  current  transformer's 
primary  E  and  protected  by  circuit  breaker  D.  The  secondary 
of  the  transformer  F  is  connected  through  the  relay  H  and  the 
trip  coil  7  on  the  oil  switch  D.  The  trip  coil  is  short-circuited 
by  the  contacts  J  and  K  on  the  relay.  An  overload  at  C  causes 


PLUNGER-TYPE  PROTECTIVE  RELAYS  43 

excessive  current  to  flow  in  the  relay  coil  H,  which  raises  its 
plunger  and  opens  the  contacts  J  and  K,  causing  current  to 
flow  through  the  trip  coil  7,  which  opens  the  oil  switch. 

One  type  of  series-trip  relay,  Fig.  44,  closes  the  contacts 
C  by  a  toggle  T,  which  is  closed  on  the  downward  travel  of 
the  plunger,  thereby  insuring  very  positive  connection.  When 
the  plunger  rises,  this  opens  the  toggle,  allowing  a  spring  to 
open  the  contacts  with  a  quick,  positive  action. 

The  series-trip  relays  have  an  advantage  in  that  they  do  not 
require  a  separate  circuit  to  trip  the  circuit  breaker.  However, 
unless  they  are  equipped  with  solidly  closed  contacts,  they  are 
liable  to  trip  the  breaker  on  a  slight  jar  or  knock. 

Figure  48  shows  the  standard  diagrams  of  connections 
furnished  by  the  Westinghouse  Co.  for  circuit  opening  relays, 
corresponding  to  the  circuit  closing  diagrams  shown  in  Fig.  40. 
The  General  Electric  Co.  furnish  the  diagrams  shown  in  Fig.  49 
for  one,  two  and  three  phase  protection  by  circuit  closing  relays. 
The  corresponding  diagrams  for  circuit  opening  relays  are  shown 
in  Fig.  50. 


CHAPTER  V 

D.C.  POWER-DIRECTIONAL  RELAYS 

During  the  early  days  of  the  electrical  industry  the  prob- 
lem of  the  protection  of  circuits  and  equipment  was  one  in 
which  the  chief  concern  was  given  to  disconnecting  the  fault 
as  quickly  as  possible.  This  generally  could  be  accomplished 
by  the  use  of  fuses  or  circuit  breakers  with  an  instantaneous 
trip.  However,  as  the  size  of  electric-power  systems  increased 
and  they  were  complicated  with  a  multiplicity  of  circuits  and 
apparatus,  and  the  necessity  of  continuity  of  service  became  an 
important  matter,  the  problem  of  protection  developed  not 
only  into  one  of  protecting  the  apparatus,  but  also  of  localizing 
the  fault  to  the  circuit  on  the  piece  of  apparatus  where  it 
occurred.  To  meet  the  various  conditions  of  protection  required 
for  machines  operating  in  parallel,  or  in  parallel  with  other  equip- 
ment, or  for  isolating  the  faulty  circuit  in  the  various  feeder 
systems,  there  have  been  developed  a  number  of  different  types 
of  relays.  Among  these  devices  the  reverse-current  relay  has, 
especially  in  the  protecting  of  direct-current  circuits  and  appara- 
tus, a  wide  application. 

One  of  the  prime  applications  of  the  D.C.  power-directional 
relay  is  for  the  prevention  of  the  reversal  and  the  discharge 
of  current  from  a  storage  battery  into  the  charging  source 
should  the  voltage  of  the  charging  equipment  fail.  In  addition 
to  this  there  are  numerous  other  applications  where  the  power- 
directional  relays  may  be  used.  Rotary  converters  operating 
in  parallel  with  a  stand-by  storage  battery  is  another  case  in 
which  a  highly  sensitive  relay  of  the  reverse-current  type  is 
required.  Or  where  rotary  converters  are  operated  in  parallel, 
if  the  alternating-current  supply  fails  on  one  machine  it  will 
be  motorized  from  the  direct-current  bus.  Even  if  the  alter- 
nating-current supply  is  interrupted  only  for  a  short  time, 
it  is  unsafe  to  run  the  converter  inverted  although  the  power 
consumed  is  very  small,  since  it  may  run  at  damaging  speed, 

44 


D.C.  POWER-DIRECTIONAL  RELAYS  45 

and  in  any  case,  were  the  alternating-current  supply  to  be 
established  after  a  very  short  interruption,  the  converter  would 
not  be  running  in  synchronism  and  might  cause  considerable 
damage.  Therefore  the  power  directional  relay  must  be  highly 
sensitive  and  trip  the  breaker  immediately  upon  a  slight  reversal 
of  power  in  the  direct-current  end. 

Figure  51  shows  a  D.C.  power-directional  (reverse-current) 
relay  with  the  cover  removed,  and  Fig.  52  is  a  diagrammatic 
scheme  of  connections. 

In  Fig.  52,  A  is  the  magnet  frame  and  B  an  iron  core  about 
which  the  moving  coil  C  is  free  to  turn  on  jeweled  bearings 
at  the  top  and  bottom  of  the  coil,  very  similar  to  the  perma- 
nent-magnet movement  voltmeter  or  ammeter.  The  field 
coil  D  is  wound  with  a  large  number  of  turns  of  fine  wire  con- 
nected directly  across  the  circuit  and  magnetizes  the  polepieces  N 
and  S.  It  will  be  noticed  that  the  potential  on  coil  D  does 
not  reverse,  no  matter  which  way  the  current  is  flowing  in  the 
circuit;  consequently,  the  polarity  of  the  magnet  is  always 
the  same.  The  movable  coil  is  connected  through  spiral  springs, 
the  same  as  a  movable  coil  in  a  direct-current  ammeter  or 
voltmeter,  to  the  shunt,  which  is  in  series  with  the  load. 
When  the  current  flows  in  the  proper  direction,  the  turning 
effort,  or  torque,  of  the  movable  coil  tends  to  keep  the  contact 
E  firmly  against  the  stop  F  and  is  also  held  in  this  position 
by  the  spiral  springs.  But  if  the  current  reverses  in  the  circuit, 
the  current  through  the  movable  coil  is  reversed,  consequently 
the  torque,  therefore  contact  F  moves  over  against  G.  This 
closes  a  circuit  to  the  shunt  trip  on  the  circuit-breaker  and  opens 
the  circuit.  Or  if  desired,  it  may  close  a  definite  time-limit 
relay,  which  in  turn  closes  the  trip  circuit.  The  position 
of  the  stop  F  and  the  contact  G  is  variable,  so  that  the  contacts 
will  not  close  until  the  load  has  reversed  to  a  definite 
predetermined  amount.  This  relay  has  a  scale  marked  in  milli- 
volts as  shown  in  Fig.  5 1,  and  may  be  set  to  act  as  low  as  2  per 
cent  reversal  of  current  or  as  high  as  100  per  cent. 

The  closing  torque  of  this  relay  is  proportional  to  the  load, 
owing  to  the  restraining  effort  of  the  spiral  springs,  consequently 
can  be  used  as  an  excess-current  relay  since  its  time  is  inversely 
proportional  to  the  excess  current.  Thus  if  the  disturbance  is 


46 


PROTECTIVE  RELAYS 


FIG.  51. — Movable-coil  type  reverse-current  relay  (Westinghouse). 


+ 

_ 

]". 

r 

SHUNT 

AAAAAA 

k 

LMB- 

\\\\\\\\\\ 

D 

u 

LINE 

Fio.  52. — Schematic  diagram  of  the  movable-coil  type  relay  shown  in  Fig.  51. 


D.C.  POWER-DIRECTIONAL  RELAYS 


47 


mild,  it  may  take  as  high  as  8  sec.  to  close,  allowing  ample  time  for 
a  transient  disturbance  to  clear  itself.  If  the  disturbance  is  more 
severe,  it  may  take  only  1,  2  or  4  sec.,  depending  on  the  violence  of 
the  disturbance,  while  on  a  dead  short-circuit  the  action  in  closing 
the  contacts  is  almost  instantaneous. 

Figure    53    shows    another   type    of   D.C.    power-directional 
relay,  which  is  not,   however,   capable  of  such  accurate  pro- 


FIG.  53. — Polarized  reverse-current  relay  ( Westinghouse) . 


FIG.  54. — Schematic  diagram  of  reverse  current  relay  shown  in  Fig.  53. 

tection.  A  schematic  diagram  of  this  relay  is  given  in  Fig. 
54.  A  is  a  permanent  bar  magnet,  with  poles  N  and  S.  Pivoted 
on  the  end  of  the  magnet  is  an  iron  armature  B  which  has  its 
ends  polarized  N'  and  Nr  by  magnetic  induction.  The  iron 
cores  of  coils  C  and  D  also  serve  to  complete  the  magnetic 
circuit  of  the  bar  magnet.  The  coils  are  wound  to  produce 
poles  N  and  S  at  the  armature  end  with  the  current  flowing 


48 


PROTECTIVE  RELAYS 


in  a  normal  direction.  Then  the  N  end  of  the  lower  coil  repels 
the  armature  N'  and  the  S  end  of  the  upper  coil  attracts  the 
armature.  This  keeps  the  contacts  E  and  F  open.  Should  the 
current  reverse,  the  polarity  of  the  electromagnets  is  re- 
versed and  the  armature  end  of  the  lower  coil  will  become 
S  polarity,  which  will  attract  the  armature  N',  while  the  upper 
coil  will  become  N  and  will  repel  the  armature  Nf;  therefore 
the  armature  moves  over  and  closes  the  contacts  E  and  F  on 


FIG.  55. — G.  E.  strap- wound  reverse- current  relay. 


reversal  of  the  current,  which  in  turn  may  close  the  circuit 
to  a  relay  switch,  definite-time-limit  relay  or  the  shunt-trip 
coil  on  the  circuit  breaker. 

The  relay  may  be  adjusted  to  operate  on  a  definite  reversal 
by  changing  the  position  of  the  stop  G  or  by  varying  the  milli- 
volts drop  across  the  relay-coil  leads.  The  millivolts  drop  may 
be  obtained  by  connecting  across  a  length  of  copper  busbar. 
Allowing  1,000  amp.  per  square  inch  of  cross-section,  6-ft. 
length  of  busbar  will  give  50  m.v.  drop.  A  correspondingly 
heavier  current  per  square  inch  will  give  the  drop  with  a  shorter 
span.  Care  must  be  taken  that  the  relay  leads  span  only  the 
solid  copper — that  is,  there  is  no  joint  included  as  a  slight 
resistance  of  a  joint  may  equal  several  feet  of  busbar  and  the 


D.C.  POWER-DIRECTIONAL  RELAYS 


49 


resistance  of  joint  is  generally  unstable.  The  drop  obtained 
in  this  manner  will  vary  greatly  with  the  heat,  as  the  resis- 
tance of  copper  increases  with  a  rise  in  temperature.  This, 
however,  should  cause  no  trouble,  as  power-directional  relays 
of  this  type  should  never  be  set  or  reliance  placed  on  a  setting 
when  a  variation  of  5  to  10  per  cent  might  mean  the  difference 
between  correct  and  incorrect  operation. 

In  Fig.  55  is  shown  a  type  of  power-directional  relay  that 
has  its  current  and  potential  coil  reversed  from  that  in  Fig. 


FIG.  56. — Relay  arranged  for  horizontal  bus. 

51.  The  exciting  coil  E  is  wound  with  heavy  copper  strap, 
and  the  potential  coil  located  in  the  upper  part  of  the  relay 
is  wound  with  fine  wire  placed  in  series  with  a  resistance  and 
connected  across  the  circuit. 

In  the  relay  shown  in  Fig.  56  the  current  coil  is  dispensed 
with,  as  the  iron  yoke  E  forms  the  winding  and  gives, enough 
magnetism  for  operation  simply  by  passing  the  circuit-breaker 
stud  or  terminal,  where  the  relay  is  mounted,  through  the  hole 
H.  Figure  57  shows  a  similar  type,  but  is  arranged  for  mount- 
ing on  a  busbar.  The  reverse-current  relay,  Fig.  58,  operates 


50 


PROTECTIVE  RELAYS 


on  a  somewhat  different  principle  from  those  in  Figs.  55,  56  and 
57.  This  is  shown  diagrammatically  in  Fig.  59.  The  iron 
magnetic  circuit  A  has  a  gap  B  on  one  side  and  an  iron  yoke 


FIG.  57. — Reverse-current  relay  arranged  (G.  E.)for  bus-bar  mounting. 


FIG.  58. — Kelay  tor  vertical  bus. 

C  across  its  center.  If  the  current  in  the  heavy  busbar  D 
is  flowing  in  the  direction  of  the  arrow,  it  will  magnetize  the 
yoke  with  a  polarity  as  shown.  The  winding  E  is  wound  with 


D.C.  POWER-DIRECTIONAL  RELAYS 


51 


a  large  number  of  turns  of  fine  wire  and  is  connected  across 
the  circuit  so  as  to  produce  a  polarity  in  core  (7,  as  indicated.  It 
is  evident  that  the  effect  of  the  current  in  the  busbar  and  that 


FIG.  59. — Schematic  diagram  of  reverse-current  relay. 


FIG.  60. — Condit  strap-wound  reverse-current  relay. 

in  coil  E  is  such  as  to  cause  a  flow  of  flux  through  the  mag- 
netic circuit  formed  by  the  magnet  A  and  the  yoke  C  of  the 
potential  coil  E,  as  indicated  by  the  dotted  line  M.  Very  little 


52 


PROTECTIVE  RELAYS 


magnetism  will  flow  through  the  part  of  the  magnetic  cir- 
cuit containing  the  air  gap  B.  Should  the  current  reverse 
in  the  busbar,  however,  the  magnetism  will  also  reverse  and 
oppose  that  of  coil  E,  but  they  will  both  unite  in  forcing  the 
magnetism  through  the  part  of  the  circuit  containing  the  air 
gap  B.  In  air  gap  B  is  an  iron  core  F,  and  when  the  magnetism 
becomes  great  enough,  the  core  will  be  attracted  upward,  thus 


FIG.  61.  FIG.  62. 

FIG.  61. — Diagram  showing  polarity  with  current  in  normal  direction. 
FIG.  62. — Diagram  showing  polarity   upon   current  reversal  and  closing  of 
contacts. 

closing  the  contacts  C,  which  are  shown  clearly  in  Fig.  58.  This 
type  of  relay  is  given  an  inverse-time  characteristic  by  equip- 
ping it  with  a  retarding  bellows  B,  which  may  be  adjusted  by 
the  air  valve  A  on  top  of  the  relay. 

Another  slightly  different  form  of  power-directional  relay 
is  shown  in  Fig.  60.  An  iron  plunger  is  used  and  two  coils 
are  placed  on  opposite  legs  of  the  iron  magnetic  circuit.  The 
arrangement  is  shown  diagrammatically  in  Fig.  61.  The 
iron  magnetic  circuit  A  and  A',  has  a  current  coil  B  on  one  leg 
and  a  potential  coil  C  on  the  other  leg,  with  a  core  D  located 
between  A  and  A ',  arranged  to  move  vertically.  With  the  current 
flowing  in  normal  direction  the  magnetism  travels  up  one  leg 


D.C.  POWER-DIRECTIONAL  RELAYS 


53 


and  down  the  other,  as  indicated  hy  the  arrows.  There  is 
no  magnetism  or  at  least  a  very  weak  field  in  the  core  D  under 
normal  load.  Should  the  current  reverse  in  B,  then  both 


FIG.  63. — Relay  arranged  for  horizontal  bus. 


FIG.  64. — Relay  arranged  for  vertical  bus. 

coils  tend  to  force  the  flux  through  the  core  D.  When  this 
reversal  is  sufficient  to  force  enough  magnetism  through  the 
core,  it  is  lifted  up  and  closes  the  contacts  E  and  F  by  the 


54  PROTECTIVE  RELAYS 

contact  disk  G,  as  in  Fig.  62,  which  in  turn  closes  the  circuit 
to  a  breaker-trip  coil. 

When  the  current  is  1,000  amp.  or  more,  the  current  coil 
is  omitted  and  a  relay  like  Fig.  63  is  used,  the  cable  or  bus 
passing  through  the  insulated  hole  H,  the  magnetic  field  set  up 
about  the  cable  being  sufficient  to  operate  the  relay.  If  the 
relay  is  to  be  used  with  a  vertical  busbar  instead  of  a  horizontal 
one,  the  parts  are  slightly  rearranged  as  shown  in  Fig.  64, 
but  the  principle  of  operation  is  the  same. 

There  are  other  relays  utilizing  similar  principles  of  operation, 
but  before  selecting  any  relay  for  a  particular  service, 
the  characteristics  of  the  relay  should  be  carefully  studied 
as  well  as  the  character  of  the  installation  to  be  protected, 
and  a  relay  chosen  which  gives  a  maximum  of  favorable  points. 
Applications  of  D.C.  power-directional  relays  will  be  fully 
discussed  in  the  next  chapter. 


CHAPTER    VI 

APPLICATIONS  OF  D.C.  POWER-DIRECTIONAL  RELAYS 

There  are  numerous  cases  in  which  power-directional  relays 
may  be  applied  not  only  to  give  complete  protection  to  D.C. 
apparatus  but  also  to  preserve  continuity  of  service  in  case  of 
failure  of  lines  or  machines.  While  their  general  use  has  been 
limited  by  the  initial  cost  and  the  relatively  small  sizes  of 
D.C.  transmission  systems  compared  to  A.C.  systems,  yet 
they  are  becoming  more  and  more  to  be  recognized  as  an  in- 
dispensable factor  in  the  correct  operation  of  any  plant.  Another 
reason  why  they  have  not  come  into  general  use  in  the  various 
plants  is  because  the  users  of  electric  energy  have  grown  into 
the  habit  of  considering  an  interruption  as  a  part  of  their  every- 
day work;  an  unavoidable  evil.  Were  the  protective  relays 
better  understood,  it  would  at  once  be  realized  that  interrup- 
tions are  not  a  necessary  evil,  but  a  large  percentage  of  the 
interruptions  on  many  systems  may  be  avoided  by  the  proper 
use  of  protective  relays. 

Storage -battery  Protection. — The  primary  application  of 
a  D.C.  power-directional  or  reverse-current  relay  is  to  protect 
a  charging  storage  battery  from  discharging  should  the  charg- 
ing source  fail.  If  the  battery  is  being  charged  by  a  small 
motor  generator,  a  failure  of  the  motor  may  cause  the  battery 
to  motorize  the  generator,  thus  exhausting  itself.  Even  if 
the  battery  is  being  charged  from  a  line  circuit  through  a  re- 
sistance, a  failure  of  the  line  will  cause  the  battery  to  discharge 
into  the  line  through  the  resistor.  If  being  charged  by  a  mechan- 
ical rectifier,  a  failure  of  the  alternating-current  may  cause 
the  battery  to  discharge  through  the  rectifier  coils.  However 
with  a  mercury-arc  rectifier  or  a  vacuum-tube  rectifier  such  as 
the  "Tungar"  or  the  "Rectigon"  the  battery  cannot  dis- 
charge in  case  of  A.C.  failure. 

Figure  65  shows  a  diagram  of  connections  giving  the  direc- 
tion of  current  in  battery,  generator  and  load.  Figure  66 

55 


56 


PROTECTIVE  RELAYS 


r 

t 

• 

\ 

TEI 

T- 

'   t 

1~ 

I 

@ 

Him 

BAT' 

Hih 

?y 

FIG.  65. — Diagram   of   connections   of   battery,   load   and   generator.     Arrows 
show  normal  direction  of  current  flow. 


t        I 


GENERATOR  7O  LOAD 

BEING  MOTORED     • 
FIG.  66. — Arrows  show   direction   of   current   flow  upon   failure   of   generator. 


F        F 


FIG.  67. — Diagram  of  connections  for  battery,  load  and  generator  for  protection 
against  current  reversal. 


APPLICA  TIONS  OF  D.C.  POWER-DIRECTIONAL  RELA  YS      57 

shows  the  direction  of  current  upon  the  failure  of  the  generator. 
It  will  readily  be  seen  that  the  battery  attempts  to  assume  the 
load  and  shows  the  correct  location  for  the  application  of  a 
reverse-current  power  directional  relay. 


FIG.   68. — Connection  diagram  of  relay  using  separate  tripping  source. 


When  the  battery  is  of  sufficient  voltage,  the  trip  circuit 
may  be  taken  directly  from  the  battery,  but  if  the  battery 
is  of  low  voltage,  the  trip  must  be  taken  from  another  source. 
Figures  67  and  68  show  diagrams  of  connections  of  a  simple 
relay  protecting  a  battery  against  reversal  of  current. 

Other  Methods  of  Protection. —  For  the  average  small  installa- 
tion, the  expense  of  a  good  relay  is  prohibitive  and  other  methods 
will  give  adequate  protection.  If  the  voltage  of  the  battery 
is  very  low,  then  a  no-voltage  release  may  be  connected  to 
trip  the  breaker  or  sound  an  alarm.  But  it  must  be  borne 
in  mind  that  if  the  voltage  of  the  battery  is  sufficiently  high 
then  if  the  charging  source  should  fail,  the  battery  will  still 
maintain  a  line  voltage  and  will  prevent  the  functioning  of 
the  no-voltage  attachment,  thus  defeating  the  purpose  for  which 
it  was  installed. 

Even  should  the  battery  be  charged  by  a  motor  generator, 
or  rotary  converter,  a  no- voltage  device  on  the  A.C.  side  must 
be  used  with  great  caution,  as  the  battery  may  motor  the  genera- 
tor and  generate  sufficient  A.C.  voltage  to  prevent  the  function- 
ing of  the  A.C.  no-voltage  device. 


58 


PROTECTIVE  RELAYS 


Other  methods  utilizing  the  no-load  and  the  reverse-current 
releases  have  been  fully  described  under  the  chapter  on  "Circuit 
Breakers  and  Releases." 

POWER-DIRECTIONAL  RELAYS  AND  STAND-BY  BATTERIES 

When  the  battery  is  large  enough  and  is  designed  to  carry 
part  or  carry  all  the  load  in  the  event  of  charging-source  failure, 
as  for  instance  in  the  case  of  a  rotary  converter  charging  the 
large  stand-by  battery  of  an  elevator  or  mine-hoist  system, 
it  is  evident  that  the  battery  must  not  be  disconnected  in  case 
of  current  reversal. 


THREt  PHASE  A.C. 


SHUNT 


'R.C.RELAY     "  •  J  T" 

TO  ELEVATOR 

MOTOR 

FIG.  69. — Connections  of  rotary  converter,  storage  battery  and  elevator  motor. 
Arrows  show  normal  direction  of  D.C.  current. 

In  order  to  determine  the  correct  point  of  application  for 
the  relays,  let  us  consider  Fig.  69,  which  shows  a  rotary  con- 
verter, feeding  a  stand-by  battery  and  the  elevator-motor  load. 


FIG.  70. — Arrows  show  direction  of  current  with  a  short  circuit  on  the  A.C.  line. 

Now  should  the  A.C.  power  fail,  due  for  instance  to  a  short 
in  the  A.C.  line  as  shown  in  Fig.  70,  at  X,  then  the  battery  will 
attempt  to  feed  the  rotary  (D.C.  end),  run  it  inverted,  generate 
alternating  current  and  feed  through  the  short  at  X.  Or  even 
if  the  A.C.  supply  is  only  interrupted  for  a  short  time,  it  would 


APPLICA TIONS  OF  D.C.  POWER-DIRECTIONAL  RELA  YS      59 

be  unsafe  to  run  the  converter  inverted  although  the  power 
it  consumes  is  very  small;  for  it  may  run  at  damaging  speed. 
In  any  case,  were  the  alternating  current  to  come  on  suddenly, 
it  would  not  be  running  in  synchronism,  and  might  cause 
considerable  damage.  Therefore,  the  reverse-current  relay 
must  be  installed  between  the  rotary-converter  and  the  load. 
It  must  be  very  sensitive  and  trip  the  breaker  immediately 
upon  a  slight  reversal  of  power  in  the  D.C.  end.  The  rotary 
must  then  be  restarted,  and  resynchronized  (if  not  of  the  self- 
synchronizing  type)  and  the  voltage  readjusted  before  recon- 
nection  to  the  D.C.  system.  This  case  calls  for  the  highest 
grade  of  relay  as  it  must  be  set  to  trip  the  breakers  instantly 
on  a  reversal  of  current  of  only  1  or  2  per  cent  of  normal  load 
current. 

If  the  battery  is  not  large  enough  to  carry  the  full  load,  then 
a  reverse-current  power-directional  relay  may  be  installed 
in  the  battery  circuit  to  limit  the  discharging  load  by  tripping 
several  of  the  unimportant  circuits.  In  this  case,  however, 
much  better  operation  is  assured  by  using  the  relay  not  to 
trip  the  circuits  directly,  but  to  sound  an  alarm,  thus  warning 
the  operator  to  pull  feeders  until  the  safe  load  of  the  battery 
is  reached  at  which  point  the  alarm  will  cease. 


I  dl    - 


LOAD 
FIG.  71. — Arrows  show  direction  of  current  with  three  generators  feeding  bus. 

Failure  of  Prime  Movers. — A  similar  case  is  where  the  gen- 
erators may  be  run  from  water  or  steam  turbines,  or  other 
type  of  engine,  where  they  may  -be  connected  in  parallel,  feed- 
ing the  same  bus,  or  charging  a  stand-by  battery.  Consider 
Fig.  71,  showing  three  generators,  each  with  its  separate  prime 
mover,  tied  into  one  bus.  Each  generator  should  be  supply- 


60 


PROTECTIVE  RELAYS 


ing  its  own  share  of  the  load.  But  suppose  the  prime  moving 
source  of  generator  No.  2  should  fail.  Being  connected  to 
a  live  bus,  the  current  would  reverse,  it  would  run  as  a  motor, 
and  keep  the  prime  mover  running  as  a  load  instead  of  a  mover, 
the  currents  being  as  shown  in  Fig.  72. 

FAIL  URE (How  being  motored) 


FIG.  72. — Arrows  show  direction  of  current  flow  upon  failure  of  one  generator. 

Suppose  the  failure  should  not  be  due  to  the  prime  mover,  but 
due  to  generator  No.  2  losing  its  field  (accidentally  disconnected 
from  exciting  circuit).  There  would  still  be  a  weak  residual 
field  upon  which  the  heavy  reverse  current  might  react,  run 
the  motor  and  prime  mover  at  terrific  speed,  breaking  the  fly- 
wheel or  doing  other  material  damage.  Still  the  current  might 
be  less  than  full-load  current. 

A  reverse-current  relay  inserted  between  the  generator  and 
protecting  breaker  would  prevent  this.  On  the  least  reversal 
of  current,  the  relays  would  quickly  close  its  contacts,  tripping 
the  breaker,  which  could  not  be  closed  until  the  conditions 
were  correct  for  normal  operation.  This  scheme  should  not 
be  depended  on  alone  to  disconnect  a  generator  with  lost  field, 
as  every  important  machine  should  be  equipped  with  a  centrif- 
ugal device  which  instantly  opens  the  circuit  in  the  event  of 
excess  speed. 

Parallel  Feeders. — Heavy  B.C.  installations  sometimes  tie 
in  a  sub  bus  with  several  tie  lines,  some  of  which  formerly  were 
used  as  spare  lines  to  be  used  only  in  case  of  emergency. 
Sometimes  important  machines  have  their  motors  fed  by 
several  feeders.  By  properly  connecting  in  reverse  power  relays 
with  overload-and-definite-time-limit  relays,  every  feeder  may 


APPLICATIONS  OF  D.C.  POWER-DIRECTIONAL  RELAYS     61 

be  used,  with  a  great  economy  in  copper,  and  a  faulty  feeder 
disconnected  immediately  without  interruption  of  service,  allow- 
ing the  other  cables  to  carry  the  load  at  overload  until  the 
damaged  line  can  be  repaired.  Figure  73  shows  the  main  bus 
tied  into  a  sub  bus  by  two  feeders  and  protected  at  the  gen- 


GENERATOR 
BUS 


I               ''•                                               ''                  ^ 

'OVERLOAD 
DEFINITE 
^^JTIME  RELAYS 

OVERLOAD 
REVERSE 

**^Q  , 

A 

SUB 
BUS 


H 


LOAD 


FIG.  73. — Normal  direction  of  current  in  parallel  feeders. 

erator  end  by  overload-and-definite-time-limit  relays,  and 
at  the  sub  end  by  reverse-current  relays.  If  a  heavy  excess 
current  occurs  on  the  sub  bus,  relays  OD  and  OD'  will  trip  the 
breaker  on  each  feeder  at  the  generating  end.  But  a  short 
on  a  bus  inside  the  station  is  of  a  very  rare  occurrence.  Suppose, 
however,  a  short  occurred  on  the  feeder  at  X,  Fig.  74.  This 


GENERATOR 


SUB 
BUS 


- 


LOAD 


FIG.  74. — Arrows  show  direction  of  current  with  a  short  circuit  on  one  feeder. 

short  will  be  fed  from  both  the  generating  end  and  the  sub  end, 
thereby  putting  a  heavy  load  or  excess  current  on  each  feeder. 
Both  relays  start  to  act  at  OD  and  OD',  but  the  current  in 
relay  Rf  has  reversed,  so  before  either  OD  or  OD'  can  trip  their 
breakers,  R'  trips  its  breakers,  thus  relieving  the  excess  current 
on  the  first  feeder.  The  excess  current  is  not  relieved  from 
relays  OD',  however,  so  in  a  second  or  so  they  trip  their  breaker, 


62  PROTECTIVE  RELAYS 

effectually  cutting  the  bad  feeder  from  service  at  both  ends  and 
still  allowing  the  first  feeder  to  feed  the  bus  load  without  inter- 
ruption. 

When  there  are  a  number  of  feeders  in  parallel,  they  may 
be  equipped  with  plain  overload  (excess-current)  relays  with 
an  inverse-time-limit.  Due  to  the  current  in  the  faulty  feeder 
being  greater  than  in  the  other  feeders,  the  inverse-time  delays 
will  usually  enable  the  relays  to  discriminate  and  trip  out  the 
faulty  feeders.  This  condition  is  practically  the  same  on  D.C. 
and  A.C.  and  is  discussed  in  detail  under  the  chapter  on  the 
" Protection  of  Parallel  Feeders  by  Excess-Current  Relays." 

D.C.  Ring  Systems. — Sometimes  there  is  an  apparatus 
which  must  be  run  without  interruption  except  in  case  of  actual 
damage  to  the  apparatus  itself,  and  even  then  the  interruption 
must  be  confined  to  the  smallest  possible  area.  To  this  end, 
spare  feeders  or  lines  are  often  run  to  each  piece  of  apparatus, 
and  in  case  of  trouble  on  one  line,  the  other  is  switched  on. 

If  the  apparatus  is  connected  in  a  ring  system,  and  power- 
directional  and  definite-time-limit  relays  supplied,  the  faulty 
feeder  or  piece  of  apparatus  may  be  automatically  cut  out 
without  interruption  to  the  rest  of  the  system.  As  shown 
in  Fig.  75,  the  tie  lines  between  the  motors  are  all  equipped  with 
relays  which  trip  only  when  the  power  flows  away  from  the 
apparatus.  Then  the  relays  on  the  side  away  from  the  main  bus 
as  at  A,  B,  C,  D  and  E  are  set  for  a  decreasing  time  element, 
as  for  instance  A  for  5  sec.,  B  for  4  sec.,  C  for  3  sec.,  and  so 
forth.  The  other  relays,  going  around  the  other  way,  F,  G, 
H,  I  and  /  are  also  set  with  decreasing  time  element,  as  F 
for  5  sec.,  G  for  4  sec.,  and  so  on.  The  feeders  are  protected 
at  the  bus  by  overload-and-definite-time  relays  having  a  time 
element  slightly  longer  than  the  longest  reverse-current  relay. 

Now,  remembering  that  a  relay  will  only  trip  when  the  current 
flows  away  from  the  sub  buses,  and  never  when  it  flows 
into  them,  consider  the  effect  of  a  short  circuit  at  X,  Fig.  76. 

An  excess  current  will  flow  in  the  direction  shown  and  relays 
A,  B,  C  and  D  will  start  operating;  also  F  and  the  main  relays 
Kr  and  K.  But  D  has  the  lowest  setting  of  any  of  the  ones  that 
start  operating.  So  at  the  end  of  2  sec.,  relay  D  trips  its  breaker, 
thus  relieving  the  excess  current  on  A,  B,  C  and  K,  which  imme- 


APPLICA TIONS  OF  D.C.  POWER-DIRECTIONAL  RELA  YS      63 


B.-4 


1*2' 


•f 


FIG.  75. — Elementary  two-wire  ring  system  for  feeding  motors  an  uninterrupted 

supply. 


•Jl 


Mr 


i*            JT 

it 

,--r<ic       _^  &>  ( 

N  31 
&' 

)          ^,. 

A 

I? 

'SHORT 


r 

4- 


Fio.  76. — Arrows  show  direction  of  current  flow  with  a  short  circuit  between 

stations. 


64  PROTECTIVE  RELAYS 

diately  reset.  The  excess  current  is  still  actuating  relays  F  and 
K,  but  as  F  is  quicker  than  K,  F  trips  its  breaker  before  K,  thus 
relieving  the  excess  current  and  K  resets. 

Thus  it  will  be  seen  that  the  faulty  line  is  disconnected  at 
both  ends  and  every  motor  is  still  running  without  interruption. 
A  disturbance  at  any  point  of  the  whole  system  will  thus  clear 
itself.  Even  if  the  motor  were  defective,  the  lines  supplying 
it  would  be  automatically  opened  on  each  end  and  would  cut 
out  only  the  defective  unit  and  allow  the  others  to  run  without 
interruption. 

To  expand  this  system  of  protection,  large  industrial  plants 
may  be  substituted  for  the  motors  and  be  fed  in  a  ring 
without  interruption,  except  to  the  plant  or  the  line  in  which 
the  disturbance  occurs. 

Or  as  a  further  expansion,  a  number  of  substations  may  be 
connected  in  a  ring  and  give  uninterrupted  service. 

Of  course,  this  is  seldom  done  on  a  commercial  scale  on 
direct-current  work,  due  to  the  expense,  unless  the  importance 
of  the  service  warrants  it,  but  the  same  system  is  widely  used 
in  high-tension  alternating-current  transmission  and  a  careful 
study  of  the  ring  system  as  applied  in  its  elementary  form 
to  D.C.  work  will  greatly  assist  in  understanding  the  action 
of  the  ring  system  in  A.C.  work  where  three  phases,  phase 
relations  and  distortions,  as  well  as  inductance  and  capacity 
effects,  must  be  considered  and  are  very  confusing  unless  the 
elementary  principle  be  clearly  understood. 

Under-current  Protection. — If  desired,  under  special  con- 
ditions, most  reverse-current  relays  may  be  adjusted  to  open 
the  breaker  when  the  load  merely  drops,  instead  of  a  full  reverse, 
by  making  the  contacts  normally  closed  and  using  the  current 
in  normal  direction  to  hold  them  open.  The  breaker  will 
trip  on  either  a  fall  in  load  or  on  reverse. 

Over-voltage  Protection. — By  suitably  changing  the  winding 
on  a  moving-coil  type  reverse-current  relay,  and  connecting 
in  series  with  a  resistor,  directly  across  the  line,  it  can  be  made 
to  give  protection  against  over-voltage.  Figure  77  gives  the 
diagram  of  connections.  As  the  voltage  rises,  it  forces  more 
and  more  current  through  the  moving  coil,  until  a  prede- 


APPLICATIONS  OF  D.C.  POWER-DIRECTIONAL  RELA  YS      65 


termined  limit  is  reached,  when  the  contacts  close  and  either 
open  a  breaker  or  ring  a  signal  bell. 

That  such  protection  is  necessary  is  shown  by  the  fact  that 
an  over-voltage  may  cause  considerable  damage  by  burning 
out  lamps  or  other  apparatus  and  still  the  total  current  might 
not  be  high  enough  to  trip  an  overload  relay.  This  is  especially 
true  of  small  plants  having  a  generator  and  prime  mover  whose 


LINE 


BATTERY  TX 
AND  BELL    (y|* 

FIG.  77. — D'Arsonval  type  relay  connected  to  ring  bell  on  overvoltage. 

speed  may  suddenly  increase,  thereby  generating  a  high  voltage 
and  doing  damage.  The  over-voltage  relay  gives  a  complete 
protection  against  this. 

Under-voltage  Protection. — By  reversing  the  leads  and  making 
the  voltage  normally  hold  the  contacts  open,  these  relays  can 
be  set  to  close  the  contacts  on  a  lower  than  normal  voltage. 
While  there  is  seldom  a  case  where  actual  damage  is  done  by 
under-voltage,  it  leads  to  other  causes,  such  as  excess  current 
due  to  a  motor  stopping  from  lack  of  voltage  to  run  it,  but  still 
having  enough  voltage  to  force  excess  current  through  it  or  to 
cause  a  reversal  of  current  as  in  the  storage  battery. 

If  the  apparatus  is  protected  against  excess  and  reverse  current 
it  is  superfluous  to  connect  a  low-voltage  relay  to  trip  the 
breaker:  but  it  is  generally  connected  to  ring  a  bell  or  give 
other  signal  that  the  voltage  is  getting  low  and  will  soon  be 
followed  by  other  disturbances  which  require  opening  the 
circuit.  This  allows  the  attendant  to  raise  the  voltage  quickly 
or  remove  the  cause  of  decrease  and  prevent  an  actual 
interruption. 


CHAPTER   VII 

INDUCTION-TYPE  CURRENT  RELAYS 

When  the  great  possibilities  of  adequate  protection  were 
seen,  and  when  it  was  realized  that  to  obtain  this  protection 
a  more  accurate  relay  than  the  plunger  type  was  required, 
the  induction-type  watt-hour  meter  was  looked  to  as  a  solution 
of  the  problem.  The  voltage  winding  of  the  watt-hour  meter 
was  displaced  by  a  current  winding,  and  contacts  were  arranged 
to  close  when  the  current  reached  a  certain  value.  The  torque, 
or  turning  effort,  of  the  disk  was  opposed  by  a  spiral  spring. 
The  magnetic  and  electric  circuits  of  a  relay  of  the  induction 
type  are  shown  in  Fig.  78.  A  is  the  main  winding,  and  under 
this  is  another  winding  similar  to  the  secondary  of  a  trans- 
former, which  supplies  the  polepiece  windings  B  and  D.  The 
path  of  the  magnetic  flux  is  indicated  by  the  dotted  lines. 

Figure  79  shows  a  standard  induction-type  relay  and  Fig. 
80  a  schematic  diagram  of  parts  as  viewed  from  the  top.  The 
disk  A  is  damped  by  the  permanent  magnets  D,  in  a  manner 
similar  to  a  watt-hour  meter,  except  that  both  windings  on  the 
electromagnet  C  operate  from  the  current  of  the  line  alone, 
as  in  an  ammeter.  Instead  of  the  disk  revolving  continuously 
when  current  is  applied,  the  rotation  is  opposed  by  the  spiral 
spring  /  fastened  with  its  outer  end  to  the  permanent  support 
E  and  its  inner  end  to  the  shaft  B,  which  also  carries  the  moving 
contact  F. 

When  sufficient  current  flows  through  the  electromagnet  C 
to  develop  in  the  disk  the  necessary  torque,  it  rotates  until 
the  contact  F  touches  contact  G,  thereby  completing  the  trip 
circuit. 

The  foregoing  describes  the  induction-type  relay  without 
its  present  refinements.  It  had  great  accuracy,  but  one  great 
drawback  was  that  the  moving  contact  "floated."  That  is, 
the  load  might  be  sufficient  to  turn  the  disk .  half  way  around, 
so  if  the  relay  was  set  to  operate  on  5  amp.,  and  the  load  was 

66 


INDUCTION-TYPE  CURRENT  RELAYS 


67 


FIG.  78. — Magnetic   and   electric     circuit    of    Westinghouse    inverse-time-limit 
induction  type  relay. 


FIG.  79. — Westinghouse  induction  type  overload  relay. 


68 


PROTECTIVE  RELAYS 


4.5  amp.  for  some  time  and  then  suddenly  increased,  the 
closing  of  the  contacts  would  be  almost  instantaneous,  because 
4.5  amp.  caused  the  contacts  to  " float"  near  to  the  tripping 
point.  To  overcome  this,  several  holes  were  cut  in  the  disk, 
beneath  the  poles  of  the  electromagnets.  These  holes  decreased 
the  torque  considerably,  but  once  the  current  became  high 
enough  to  move  the  disk  slightly,  the  holes  were  moved  out 
from  under  the  poles,  and  the  latter  then,  acting  on  the  solid 
metal,  caused  the  disk  to  revolve  until  the  contacts  were  closed. 


FIG.  80. — Showing  position  of  contacts,  magnets,  etc.  in    Westinghouse    relay 

(top  view). 


This  insured  an  inverse  time  limit  on  all  overloads,  as  the  disk 
was  always  at  its  starting  point  until  an  overload  occurred. 
As  all  circuits  could  not  be  set  to  trip  on  the  same  overload, 
taps  were  brought  out  on  the  current  coil,  which  enabled  the 
operating  current  to  be  varied  over  a  wide  range,  a  common 
range  being  4,  5,  6  ,7  and  8  amp.,  although  later  practice  has 
often  shown  4  to  12  or  4  to  16  amp.  preferable. 

In  the  induction-type  relay,  Fig.  79,  the  taps  are  changed 
by  inserting  a  screw  in  a  marked  plate,  to  make  contact  with 
the  desired  tap.  The  metal  piece  on  the  front  of  relay,  Fig. 
81,  has  a  number  of  tapped  holes  to  receive  the  screw,  which 
may  be  put  in  any  hole  and  make  contact  with  the  taps.  The 
main  coil  is  wound  on  the  electromagnet  and  has  the  taps 
brought  out  at  the  correct  turns  to  give  the  desired  operation. 


INDUCTION-TYPE  CURRENT  RELAYS 


69 


The  block  is  of  insulating  material  and  is  arranged  so  the  taps 
cannot  pull  out. 

As  has  been  pointed  out  previously,  it  is  sometimes  desirable 
to  have  an  inverse-time  limit  on  moderate  overloads  and  a 
definite-time  limit  in  the  case  of  severe  short-circuits.  This 


FIG.  81. — Current  tap  plate  of  Westinghouse  induction  overload  relay. 


combination  of  inverse  time  on  moderate  overload,  gradually 
decreasing  to  a  definite  time  on  heavy  overloads  or  short-circuits, 
is  met  in  the  induction-type  relay  by  the  use  of  a  small  trans- 
former called  a  "torque  compensator." 

Torque  Compensators. — This  torque  compensator  is  intro- 
duced into  the  secondary  circuit  as  shown  in  Fig.  82.  The 
main  winding  A  carries  the  main  current  as  in  Fig.  78  and, 
by  its  transformer  action,  induces  a  current  in  the  primary 
B  of  the  small  transformer.  On  the  opposite  side  of  the  core 
C  is  the  secondary  D  which  supplies  the  polepieces  E  and  Ef 


70 


PROTECTIVE  RELAYS 


with  the  necessary  current  to  react  on  the  main  flux,  or  magnet- 
ism, from  the  pole  F  produced  by  coil  A.  At  rroderate  loads 
the  current  in  D  will  increase  in  proportion  to  B,  but  the  iron 
in  C  is  of  a  cross-section  such  that,  after  the  current  in  coil 
B  passes  a  certain  value,  the  core  becomes  saturated,  conse- 
quently the  current  in  D  cannot  increase  no  matter  what  the 
overload  may  be.  Since  the  current  in  the  coil  cannot  increase 
above  a  definite  value,  the  torque  on  heavy  overloads  becomes 
constant,  thus  resulting  in  a  definite-time  delay. 


FIG.  82. — Magnetic    and    electric    circuit    of    Wcstinghouse    definite-minimum 

time  limit  relay. 

It  is  essential  in  the  protection  of  radial  and  ring  systems 
that  the  time  of  the  relay  be  variable.  To  accomplish  this, 
the  angle  through  which  the  contact  must  travel  is  varied. 
Thus,  if  the  contact  must  travel  one-half  revolution,  it  may 
take  2  sec.,  but  if  a  stop  H,  Fig.  80,  is  arranged  so  that  the 
contact  need  make  only  one-fourth  revolution,  the  time  may 
be  halved.  Other  positions,  easily  set  by  a  small  lever,  enable 
any  time  from  instantaneous  to  a  maximum  to  be  readily  set. 

A  typical  curve  for  an  overload-induction  relay  is  given 
in  Fig.  21,  reference  to  which  will  show  that  150  per  cent  load 
takes  about  10  sec.  for  the  relay  to  close  its  contact;  200  per 
cent  load  requires  about  5  sec.;  500  per  cent,  2. 75  sec.;  1,000 
per  cent  and  any  overload  in  excess  of  this  take  a  definite 
time  of  about  2  sec.  These  values  are  taken  at  the  highest 


INDUCTION-TYPE  CURRENT  RELAYS  71 

time  setting;  if  the  setting  is  halved,  the  time  is  halved.  In 
other  words,  the  time  is  almost  proportional  to  the  lever  settings. 

Relay  Contacts. — Owing  to  the  accuracy  required  in  a  pro- 
tective relay,  the  parts  must  be  small  and  delicately  constructed. 
Consequently  the  contacts  of  the  trip  circuit  cannot  be  large  and 
bulky,  and  are  not  designed  to  open  the  trip  circuit  either  in- 
tentionally or  unintentionally,  once  it  has  been  established. 
This  is  one  point  that  requires  careful  consideration  in  the  design 
or  selection  of  a  relay,  as  the  tripping  circuits  are,  as  a  rule, 
highly  inductive,  and  an  arc  which  would  naturally  follow  the 
opening  of  the  circuit  might  persist  for  a  considerable  length  of 
time  and  result  in  serious  burning  of  contacts.  For  this  reason 
it  is  necessary  that  the  tripping  circuits  be  opened  by  an  auxiliary 
pallet  switch  or  contacts  fastened  to  the  circuit  breaker  in  such  a 
manner  that  the  opening  of  the  breaker  automatically  opens  the 
tripping  circuit. 

Contactor  Switches. — There  are  two  reasons  why  the  contactor 
switch  is  required.  In  the  first  case,  take  for  instance  a  circuit 
breaker  which  requires  0.2  of  a  second  to  open  after  the  relay 
contacts  have  closed  and  the  trip  coil  of  the  breaker  has  been 
energized.  Suppose  that  the  overload  on  the  line  should  dis- 
appear in  the  small  interval  between  the  instant  of  contact  closing 
and  the  opening  of  the  breaker.  The  relay  would  instantly 
attempt  to  reset  and  in  doing  so  would  open  the  circuit  before 
the  auxiliary  pallet  switch  opened  it,  thus  resulting  in  severe 
contact  burning.  The  contactor  switch  overcomes  this. 

In  the  second  case,  the  overload  might  just  be  great  enough 
to  barely  close  the  contacts.  This  weak  closing  might  not  allow 
sufficient  current  to  pass  through  to  operate  the  trip  coil  of  the 
breaker  and  the  contacts  would  "chatter"  and  burn  badly. 

To  overcome  this,  the  contactor  switch  may  be  employed  to 
change  a  weak  fluttering  contact  into  a  good  positive  contact 
which  will  keep  the  trip  circuit  closed  in  the  relay  until  it  is 
opened  externally  by  the  pallet  switch  on  the  breaker. 

A  diagrammatic  scheme  of  the  old  style  contactor  switch  is 
shown  in  Fig.  83  while  a  diagram  of  the  modern  contactor  switch 
is  shown  in  Fig.  84.  In  both  these  figures,  the  main  relay  con- 
tacts A ,  when  they  close  the  tripping  circuit  of  the  relay,  energize 
a  small  coil  B  that  attracts  an  iron  armature  or  plunger  C  and 


72 


PROTECTIVE  RELAYS 


closes  the  contacts  D,  which  are  in  parallel  with  the  main  con- 
tacts A.  Thus  it  will  be  seen  readily  that  even  though  the 
contacts  A  should  open,  the  current  will  pass  through  B  and  con- 


K     ifii      1      M«       i  1  f 
x/V/v'  \x\/'     ' 

1  c 

. 

F1 

-^  

< —  Tripping  Circuit---  H 
FIG.  83. — Schematic  diagram  of  obsolete  contactor  switch. 

tacts  D  will  stay  closed  until  the  pallet  switch  on  the  breaker 
opens  the  circuit;  when  this  occurs  coil  B  will  lose  its  pull  and 
open  contacts  D.  This  contactor  switch  will  close  about  20 
amp.  at  220  volts. 

rMain  Relay  Contact  (A) 


Relay 
Terminals 
jL 

1 

-           0- 

—0  

/•>           jT-V--^- 

*** 

r= 

spT  f 
^-;::i 

-/ro/7  Plurrger(C) 
-Coil(B) 

..•Terminal 

m 

^r 
^^ 

w 

/"y           y^y  

^>                                  ^//      ^^                                      &* 

5  ilver'Contacfs(D)  1.  lr,s^a^d  Silver  DM 
FIG.  84. — Schematic  diagram  of  modern  contactor  switch. 

In  Fig.  85  is  shown  a  view  of  the  assembled  contactor  switch 
and  also  an  exploded  view  showing  the  plunger  and  contacts. 
This  contactor  switch  is  usually  mounted  in  the  bottom  of  the 
relay  as  shown  in  Fig.  86. 

If  the  trip  circuit  requires  more  than  the  current  which  can  be 
safely  handled  by  the  contactor  switch,  then  an  auxiliary  relay 


INDUCTION-TYPE  CURRENT  RELAYS 


73 


FIQ.  85. — Assembled   and   exploded    view   of   Westinghouse    contactor   switch. 


FIG.  86. — Showing  contactor  switch  and  curve  plate. 


74 


PROTECTIVE  RELAYS 


such  as  shown  in  Figs.  195  and  196  may  be  used.  The  auxiliary 
relay  shown  in  Fig.  197  not  only  handles  greater  current,  but 
may  also  be  arranged  to  trip  several  circuits  upon  the  function- 
ing of  one  relay. 

Continuity  Indicator. — One  difficulty  frequently  encountered 
is  the  burning  out  of  trip-current-carrying  springs  due  to  excessive 
trip  currents  or  contact  arcing.  To  overcome  this,  special  con- 
tacts may  be  arranged  as  in  Fig.  87  to  prevent  the  spring  from 
carrying  any  current.  Contact  A  is  stationary  while  B  is 

D.C. 
Vo/tmeter 


FIG.  87. — Special    arrangement    of    contacts    when    required    with    continuity 

indicator. 

mounted  on  a  thin  flexible  strip  C.  Then  when  the  disk  turns, 
the  arm  D  strikes  the  contact  B,  forcing  it  against  A  and  com- 
pleting the  trip  circuit. 

Some  companies  connect  a  small  voltmeter  or  a  pilot  lamp 
across  the  contacts  as  in  Fig.  87  in  order  to  tell  by  its  continuous 
indication  that  the  trip  circuit  is  alive  up  to  the  contacts.  Should 
the  meter  fail  to  indicate  or  the  pilot  lamp  go  out,  the  operator 
knows  immediately  that  the  trip  circuit  has  failed.  In  other 
cases,  a  small  telegraph  relay  is  connected  to  ring  a  bell  when  the 
circuit  fails. 

Another  later  form  of  continuity  indicator  or  "supervisory 
circuit"  is  formed  in  this  relay  by  adding  a  second  spiral  spring 
and  using  the  standard  relay  with  its  current-carrying  spring  and 
moving  contacts.  The  voltmeter  or  pilot  lamp  is  connected  to 


INDUCTION-TYPE  CURRENT  RELAYS 


75 


feed  through  both  springs,  so  if  the  main  spring  burns  out,  then 
the  continuity  indicator  indicates  by  the  pilot  lamp  going  out. 
The  General  Electric  Induction  Relay. — A  front  view  of  this 
relay  is  shown  in  Fig.  88  where  the  external  similarity  to  the 
house-type  watt-hour  meter  is  readily  apparent.  From  the 
schematic  diagram  of  the  front  mechanism  as  shown  in  Fig.  89 
it  will  be  seen  that  there  is  a  disk  A  which  is  driven  by  a  U-shaped 


FIG.  88. — The  General  Electric  Co.  induction  type  overload  (excess  current) 

relay. 


electromagnet  (not  shown)  and  which  is  damped  by  the  per- 
manent magnets  C.  Upon  the  occurrence  of  an  overload  the 
electromagnet  causes  the  disk  to  turn  against  the  restraining 
action  of  the  spring  U.  It  will  be  noted  that  the  edge  of  the  disk 
is  slotted  with  slots  of  decreasing  depth,  so  that  as  the  disk  re- 
volves, more  and  more  metal  is  placed  under  the  action  of  the 
driving  magnet,  thus  resulting  in  an  increased  torque  which 
offsets  the  increasing  restraining  action  of  the  spring  and  prevents 
the  disk  from  floating. 


76 


PROTECTIVE  RELAYS 


Contact  Mechanism 


FIG.  89. — Schematic  diagram  of  G.  E.  induction  type  overload  relay. 


INDUCTION-TYPE  CURRENT  RELAYS 


77 


The  pinion  S  causes  the  gear  to  revolve,  and  as  soon  as  the 
disk  revolves  far  enough,  a  pin  on  gear  T  pushes  the  contacts 
D  together,  thus  completing  the  tripping  circuit. 

In  order  to  prevent  the  contacts  from  fluttering  or  opening 
the  trip  circuit  and  causing  the  contacts  to  burn,  the  electro- 
magnet G  is  connected  in  series  with  the  trip  circuit  and  arranged 
so  that  the  first  flutter  of  current  which  passes  through  the  trip 
coil  energizes  the  electromagnet,  which  quickly  attracts  the 
iron  armature  on  contacts  D  and  holds  them  positively  shut  until 
the  trip  circuit  is  opened  by  the  auxiliary  pallet  switch  on  the 
circuit  breaker. 

Time -Load  Curves. — It  has  previously  been  shown  why  a 
definite  or  inverse-definite  time  delay  was  preferable  to  a  true 
inverse  time  delay  where  the  curves  may  intersect  at  extremely 


OTRIP 


S  0.4  0.8     I.I  1.5  2.0  2.5 

[WES   3  0.3  0.6  0.8  f.l  1.4  1.8 

"    T  5  0.3  0.5  0.7  0.9     I.I  I, 

TAP    10  QJ  0.4  0.6  0.7  0.9  U 

:TTiK520  0.2  0.4  0.5  0.7  0.8  1.0 

30  0.8  0.3  0.5  0.6  O.B  0.9 

HO  n?  n.a  n  A.  B.R  a  7  o,a 


1.7  2.3    2,3    3.7    47    5.9    7.g    B.B| 

I.I      1.5    2.0    2.5    3.1     3.9    4.6    5.6 

1.8  I.I      1.4     I.B    2.S    2.7    3.2    4.0 


FIG.  90. — Index  plate  of  G.  E.  induction  overload  relay. 

heavy  overloads.  In  this  relay,  this  inverse-approaching-definite 
minimum  delay  is  obtained  by  using  a  small  saturation  trans- 
former connected  so  that  its  primary  carries  the  line  current  and 
its  secondary  feeds  the  driving  electromagnet.  At  high  currents, 
this  transformer  limits  the  current  supplied  to  the  driving  electro- 
magnet and  results  in  curves  as  shown  in  Fig.  22.  It  will  be 
noted  that  although  the  time  is  slightly  inverse  throughout  the 
entire  length  of  the  curve,  yet  the  curves  never  intersect  and  are 
clearly  distinguishable  even  at  5,000  per  cent  of  normal  load. 
This  overload  is  seldom  met  in  actual  practice  except  on  very 
exceptionally  heavy  short-circuits. 

Practice  has  shown  on  this  relay  that  a  table  of  figures  is 
preferable  and  more  easy  to  interpret  than  a  set  of  curves.  Con- 
sequently, the  relay  is  provided  with  a  tabulated  nameplate 


78 


PROTECTIVE  RELAYS 


as  in  Fig.  90  which  is  self-explanatory  from  a  careful  study.  If 
desirable,  a  transcript  plate  may  be  made  in  which  the  actual 
transformer  secondary  operating  currents  are  tabulated  instead  of 


AMP.  RA 

TING  OF  TAP 

TIME    IN    SECONDS 

s 

10 

1 

3ft 
& 

13 

6 

7.5 

9 

12 

L5 

0.9 

1.4 

2.0 

2.6 

3.2 

3.7 

4.3 

5.1 

W 

7.0 

2 

£ 

10 

12 

16 

20 

0.8 

1.1 

1.5 

2.0 

2.4 

2.9 

3.3 

3,S 

4.4 

5.2 

3 

12 

15 

IS 

24 

"50 

0.6 

0.9 

1.2 

1.5 

1.3 

2.1 

2.5 

2.9 

3.3 

3.9 

5 

20 

25 

50 

40 

50 

0.5 

0.7 

0.9 

1.2 

1.5 

1.7 

2.0 

2.3 

2.7 

3.1 

10 

40 

50 

60 

60 

100 

0.4 

0.6 

0.6 

1.0 

1.2 

1.4 

1.6 

1.9 

2.2 

2.5 

bo 

go 

100 

120 

160 

200 

0.4 

0.5 

0.7 

0.9 

1.1 

1.3 

1.5 

1.8 

1-9 

2.2 

^.h 

po 

120 

150 

100 

240 

}00 

0.3 

0.4 

0.6 

0.8 

1.0 

1.2 

1.4, 

1.6 

1.8 

2.1 

50 

200 

250 

300 

400 

500 

03 

0.4 

0.6 

0.7 

C.9 

1.1 

1.3 

1.5 

1.7 

2.0 

SECONDARY  CURRENT 
IN  RELAY 

1 

2 

3 

4- 

5 

6 

7 

8 

9 

10 

LEVER     SETTING 

FIG.  91. — Operator's  transcript  of  index  plate. 

using  multipliers.  Such  a  transcript  is  shown  in  Fig.  91  and  is 
often  of  great  convenience  when  changes  in  settings  must  be 
quickly  made. 


Current  Tap 

PluqinlOAmp. 

Hole 


FIG.  92. — Location  of  the  current  tap  plate  in  the  G.  E.  induction  overload  relay. 

Time  Settings. — The  time  is  readily  controlled  by  a  small 
lever  E  (Fig.  89)  which  moves  over  a  divided  scale  N.  For 
instance  if  the  relay  takes  2  seconds  at  a  certain  load  with  No.  10 


INDUCTION-TYPE  CURRENT  RELAYS  79 

setting,  then  at  the  same  load  it  will  take  1  second  at  No.  5 
setting,  or  0.2  seconds  at  No.  2  setting,  etc. 

Current  Tap  Plate. — A  close-up  view  of  the  current  t'ap  plate  is 
shown  in  Fig.  92.  This  plate  contains  taps  from  the  primary 
coil  of  the  saturation  transformer  and  is  arranged  to  keep  the 
ampere  turns  constant.  In  changing  taps,  an  extra  plug  is 
screwed  into  the  desired  hole  and  then  the  first  one  removed. 
Two  plugs  must  never  be  left  in  at  one  time  as  this  would  short- 
circuit  part  of  the  transformer  primary;  nor  must  both  plugs  be 
withdrawn  at  once  as  this  would  open-circuit  the  series  line 
transformer  and  might  result  in  a  dangerously  high  potential 
at  the  relay  terminals.  The  numerals  4,  5,  6,  8,  and  10  represent 
the  minimum  values  of  current  in  amperes  that  each  tap  requires 
to  cause  the  relay  to  close  its  contacts. 

Adjustment  of  Tripping  Current. — Although  every  relay  as  it 
leaves  the  manufacturer  is  carefully  adjusted  to  trip  on  its  rated 
current,  yet  due  to  variation  in  wave  form,  or  frequency,  or 
shocks  in  transportation  it  will  sometimes  be  found  that  the  relay 
requires  slight  adjustment.  For  this  purpose,  a  flux  shunting 
screw  is  conveniently  located  on  the  lower  left  hand  side. 
Loosening  the  lock  nut  and  turning  the  screw  to  the  right  in- 
creases the  current  required  and  turning  to  the  left  decreases  it. 
After  adjustment  the  screw  should  again  be  locked  by  the  nut. 

Relation  of  Various  Parts. — When  the  relay  has  been  repaired 
or  reassembled  it  is  necessary  to  see  that  the  first  short  slot  in  the 
edge  of  the  disk  stands  just  under  the  front  edge  of  the  opening 
in  the  left-hand  frame.  This  is  to  insure  against  "floating"  of 
the  disk  at  low  overloads. 

The  time  lever  must  be  set  on  zero  and  then  the  contacts 
adjusted  so  they  are  barely  closed.  When  free,  these  contacts 
should  be  separated  from  each  other  by  about  %4  inch. 

In  these  relays  will  be  found  a  black  spot  painted  on  the  edge 
of  the  disk  (as  in  Fig.  93)  and  this  spot  should  come  exactly  in 
the  center  line  of  the  bracket  which  supports  the  permanent 
magnets. 

With  the  time  lever  set  on  zero  of  its  scale  the  contact  mechan- 
ism is  brought  into  position  for  assembly  with  the  relay  frame, 
and  the  holding  screws  partially  set  up,  leaving  the  gear  and 
pinion  disengaged.  The  disk  is  then  rotated  carefully  in  a 


80 


PROTECTIVE  RELAYS 


counterclockwise  direction  (looking  down)  from  its  free  position 
through  approximately  %  of  a  revolution  until  the  spot  painted 
on  the  edge  of  the  disk  is  midway  between  the  pole  tips  of  the 
permanent  magnet.  The  gear  and  pinion  are  then  engaged  and 
the  mechanism  secured  by  tightening  the  holding  screws.  Care 
must  be  taken  that  the  gears  are  not  meshed  too  deeply.  There 
should  be  a  little  play  in  them  when  the  disk  is  held  fixed  and  the 
gear  wheel  shaken  back  and  forth. 


Contacts 
Closed 


Time  Lever 
onO 


Slots  in 
Disk 

Black  Spot 
on  Center 
of  Maqneti. 
Supports 

FIG.  93. — Showing  the  correct  relation  between  contacts,  time  lever  and  black 
disk  spot  in  the  G.  E.  induction  overload  relay. 


The  correct  location  of  the  driving  magnets  is  also  important. 
Both  the  upper  and  lower  pole  pieces  have  a  secondary  conductor 
or  shading  ring  and  the  angular  position  of  the  pole  pieces  with 
respect  to  the  radius  of  the  disk  at  their  center  determines  in 
a  large  degree  the  torque  exerted  by  the  disk  when  a  given  current 
is  applied  to  the  relay  windings. 

The  upper  pole  piece  is  secured  to  a  pin  in  the  U-shaped 
driving  magnet  in  such  a  manner  that  it  can  rotate  slightly  as  the 


INDUCTION-TYPE  CURRENT  RELAYS  81 

temperature  compensating  strip  is  attached  to  it  and  thus  is  en- 
abled to  keep  the  torque  constant  regardless  of  changes  in 
ambient  temperatures.  The  compensating  strip  may  also  be 
moved  if  desired  by  means  of  the  "  Temperature  Compensator 
Screw"  shown  in  Fig.  94. 

The  lower  pole  piece  is  rigidly  secured  to  the  driving  magnet 
and  this  adjustment  should  not  be  disturbed. 


Ijsmperature  Compensator  Screw 

FIG.  94. — Showing  the  position  of  the  temperature  compensating  screw  in  the 
G.  E.  induction  overload  relay. 

The  permanent  magnets  on  the  front  of  the  relay  serve  as  a 
damping  or  retarding  element  for  the  disk  and  they  may  be  moved 
in  or  out  to  vary  the  time  delay  of  the  relay.  The  general  prac- 
tice is  to  set  these  magnets  so  that  when  the  time  lever  is  set  on 
10,  and  applying  80  amp.  to  the  4-amp.  tap,  there  will  be  a 
time  delay  of  2.2  sec. 

If,  for  any  reason,  these  magnets  must  be  removed,  a  mark 
should  first  be  scribed  on  the  face  of  the  bracket  to  act  as  a 
guide  in  accurately  relocating  the  magnet,  when  it  is  replaced. 

It  is  important  to  note  in  changing  magnets  that  they  should 
never  be  set  out  so  far  that  the  inner  end  of  the  slots  in  the 
disk  will  pass  at  any  part  of  the  disk  travel  very  near  to  the 
outer  edge  of  the  permanent  magnets.  If  set  further  out  than 
this,  the  tune  of  operation  will  be  decreased  instead  of  made 
longer.  In  other  words,  the  point  of  greatest  retardation  is 
obtained  when  the  outer  edge  of  the  permanent  magnet  is 
about  J£  inch  inside  of  the  lower  end  of  the  slots  in  the  disk. 


82  PROTECTIVE  RELAYS 

When  replacing  the  time  index  plate  be  sure  that  it  is 
adjusted  so  that  its  back  does  not  touch  the  edge  of  the  disk. 

INDUCTION  VS.  SOLENOID-PLUNGER  RELAYS 

There  are  a  number  of  disadvantages  in  the  solenoid-plunger 
relays  which  are  not  present  in  the  induction  type.  In  the 
air-bellows  lagged  type,  the  time  is  very  inaccurate  and  unreli- 
able, due  to  the  drying  out  of~the~Ieather. 

Another  difficulty  is  that  the  continuous  vibration  to  which 
they  are  subjected  gradually  loosens  the  nuts,  screws,  etc., 
unless  the  relay  is  unusually  well  built.  The  noise  is  also  objec- 
tionable. The  force  on  the  plunger  increases  as  the  square  of 
the  current,  with  the  result  that  the  forces  reach  such  enormous 
values  during  a  heavy  overload  that  the  leather  may  be  stretched 
or  even  burst.  It  is  no  uncommon  thing  for  relays  of  this 
type  to  be  so  badly  damaged  that  they  will  fail  to  operate 
the  next  time  another  short-circuit  occurs. 

The  definite-time-limit  relays  are  not  subject  to  such  defects 
in  the  bellows  due  to  overload,  but_their  inherent  variation 
in  time  makes  them  unreliable  for  selective  action  closer  than 
about  one  second.  Another  disadvantage  of  bellows  type 
relays  is  that,  once  the  core  is  lifted,  then  in  order  to  reset,  the 
current  must  drop  to  40  or  50  per  cent  of  the  minimum  tripping 
value. 

The  oil-damped  relays  are  not  at  all  permanently  accurate 
and  change  in  time  greatly  due  to  changes  in  the  viscosity  of 
the  oil  upon  changes  in  temperature. 

An  objection  to  the  use  of  all  solenoid-plunger  relays  is  that 
;  the  expense  of  adjusting  them  for  accurate  work  is  often  greater 
than  the  cost  of  the  relays  themselves.  It  is  possible  that  an 
automatic  sectionalizing  scheme  could  be  laid  out  so  that  time 
limits  varying  by  steps  of  1  to  2  sec.  could  be  used,  in  which  case 
the  bellows  type  of  relay  might  be  sufficiently  accurate,  but  such 
accuracy  could  not  be  obtained  except  at  considerable  expense. 
In  order  to  adjust  relays  of  this  type  it  is  generally  necessary 
to  disconnect  them  from  the  circuit  and  connect  them  to  a  test 
circuit,  which  in  many  cases  is  not  easy  to  obtain.  In  addition, 
a  chronograph,  ammeter  and  control  device  are  necessary. 
Needless  to  say,  such  a  calibration  must  be  made  by  a  skilled  . 


INDUCTION-TYPE  CURRENT  RELAYS  83 

tester.  If  a  change  in  the  time  limit  is  later  required  it  is 
necessary  to  repeat  the  entire  process. 

The  best  feature  of  the  induction  type  of  overload  relay  is 
its  remarkable  accuracy  and  permanence  of  calibration.  The  use 
of  permanent  magnets  as  a  time-limit  device  prevents  overswing- 
ing  and  chattering  of  the  contacts,  and  the  construction  is  such" 
that  the  relay  will  instantly  cease  its  movement  when  the  over- 
load disappears.  There  is  no  possibility  of  mechanical  injury 
due  to  excessive  currents  when  the  torque  compensator  is  used, 
because  the  saturation  of  the  iron  prevents  the  mechanical 
forces  from  increasing  beyond  a  certain  amount. 

The  current  and  time  adjustment  of  the  induction  relays 
are  plainly  and  accurately  marked  and  any  desired  change 
can  be  made  at  a  moment's  notice.  This  is  a  feature  much 
appreciated  by  the  operating  man  who  is  responsible  for  the 
successful  operation  of  the  automatic-sectionalizing  devices 
on  his  system.  He  can  personally  check  the  setting  of  every 
relay  and  thus  be  sure  that  no  incorrect  operation  will  result 
due  to  the  carelessness  or  incompetence  of  an  assistant. 

Load  on  Instrument  Transformer. — When  selecting  a  relay 
for  use  on  current  transformers  which  also  operate  instruments, 
it  is  important  to  consider  the  load  which  the  relay  places  on 
the  transformer.  The  induction  type  of  relay  requires  a  smaller 
amount  of  energy  than  does  any  other  type,  a  feature  to  be 
appreciated  when  bushing- type  current  transformers  are  used. 

Relay  Specifications. — In  order  that  unreliable^ -and  unsat- 
isfactory overload  and  underload  relays  may  not  be  used  in 
installations,  it  is  always  well  to  add  the  following  specifica- 
tions. If  a  relay  meets  these  fundamental  requirements  and 
is  well  constructed,  it  should  be  satisfactory,  but  these  speci- 
fications will  bar  the  undesirable  relays.  ^ 

"  Overload-protective  relays  shall  be  equipped  with  a  time 
limit  that  varies  inversely  with  the  current  at  all  moderate 
overloads  and  which  will  not  drop  below  a  definite  minimum 
time  at  extreme  overloads.  The  definite  minimum  time  limit 
shall  be  adjustable  for  all  values  between  0  and  2  sec.  (or  0 
and  4  sec.),  which  adjustment  shall  be  accurate  and  permanent. 
The  relays  shall  be  calibrated  at  the  factory,  and  the  calibrating 
data  shall  be  fixed  to  the  front  of  the  relay.  It  shall  be  possible 


84  PROTECTIVE  RELAYS 

to  make,  without  the  use  of  any  testing  equipment  or  timing 
devices,  independent  adjustment  of  both  the  time  limit  and 
the  overload  value  at  which  the  relay  will  operate.  Relays  shall 
be  so  constructed  that  they  will  not  be  damaged  or  their  calibra- 
tion affected  by  the  maximum  current  that  the  generating  equip- 
ment can  deliver  to  them.  Their  construction  shall  be  such 
that  in  case  an  overload  ceases  before  the  relay  contacts  have 
been  closed,  the  relay  will  instantly  commence  to  reset  to  its 
starting  position.  The  energy  that  the  current  transformer 
must  furnish  to  operate  a  relay  shall  not  be  in  excess  of  20 
volt-amp." 


CHAPTER  VIII 


A.C.  POWER-DIRECTIONAL  RELAYS 

In  the  transmission  of  electric  energy,  there  is  perhaps 
no  more  important  piece  of  apparatus  than  the  power-directional 
relay  which  is  used  to  discriminate  or  localize  and  isolate  a  defec- 
tive feeder  or  substation  and  thereby  secure  a  maximum  of 
continuous  service.  These  relays  are  frequently  called  "  reverse- 
current"  relays,  "  overload  and  reverse-current"  relays,  "  re  verse- 
power"  relays,  and  ''reverse-overload  relays."  *  Although  quite 
good  protection  has  been  obtained  by  the  use  of  split-conductor 
and  pilot-wire  systems  for  the  protection  of  parallel  feeders, 
(as  will  be  described  later),  yet  such  systems  are  very  expensive 


FIG.  95. — Showing  why  the  current    reverses  when    one   "parallel  feeder"  is 

shorted. 

to  install  and  maintain.  Their  greatest  excuse  at  the  time 
of  installation  was  the  lack  of  a  reliable  power-directional 
relay  as  it  must  be  admitted  that  the  early  relays  were  quite 
lacking  in  some  points,  and  since  the  perfection  of  the  present- 
day  power-directional  relays,  the  pilot-wire  and  split-conductor 
systems  are  used  only  on  certain  systems,  where  careful  design 
and  calculation  indicates  a  distinct  advantage. 

The  development  of  a  satisfactory  power-directional  relay, 
however,  was  by  no  means  a  simple  proposition.  In  order  to 
better  illustrate  this,  it  may  be  well  to  consider  some  of  the 
early  forms  and  show  why  they  failed. 

85 


86  PROTECTIVE  RELAYS 

A  review  of  the  most  common  use  of  the  power-directional 
relay  is  shown  in  Fig.  95,  where  A  is  a  generator  feeding  the  bus 
B,  which  supplies  the  sub-bus  C  over  the  parallel  tie  lines  D 
and  E.  Suppose  a  short-circuit  occurs  at  X  on  feeder  D.  Cur- 
rent will  feed  into  the  "short'7  X,  directly  from  the  bus  B  and 
also  over  the  line  E,  through  C  and  into  X.  But  it  will  be 
noted  that  while  the  power  flow  in  feeder  E  and  in  feeder 
D  as  far  as  X  is  in  the  normal  direction,  yet  the  power  flow  in  the 
section  of  the  feeder  D  between  X  and  C  has  reversed.  Or 
in  other  words  the  current  (instantaneous  values)  has  re- 
versed its  polarity  with  respect  to  the  voltage  (instantaneous 
values).  This  reversing  of  respective  instantaneous  polarities 
of  current  and  voltage  is  responsible  for  the  term  "reverse 
current. " 

It  might  appear,  at  first  thought,  an  easy  matter  to  place 
contacts  on  a  wattmeter,  which  would  hold  open  on  normal 
direction,  and  close  on  reversal  of  power.  But  should  the 
"short"  be  near  the  substation,  the  voltage  will  be  very  low, 
although  the  current  may  be  high,  and  the  power  actuating 
the  wattmeter  will  be  extremely  low  in  this  case.  In  fact, 
in  some  tests  made,  where  the  line  was  actually  shorted  inten- 
tionally, it  was  shown  that  the  voltage  may  drop  as  low  as 
1  per  cent  of  normal. 

Further,  we  are  usually  dealing  in  actual  practice  with  three- 
phase  current,  and  phase  distortions  must  be  considered,  par- 
ticularly in  the  case  of  short-circuits  from  one  line  to  ground,  or 
on  one  phase  only.  These  short-circuits  may  so  distort  the  rela- 
tion of  current  to  voltage  as  to  cause  the  angle  between  them  to 
be  almost  90  deg.,  and  consequently  there  is  the  worst  condition 
for  low  torque  in  the  wattmeter  element,  i.e.  very  low  power 
factor  and  very  low  voltage. 

Still,  one  of  the  first  relays  used  for  reverse-power  trip- 
ping had  a  wattmeter  element  which  closed  contacts  on  reversal 
of  power.  In  this  form,  the  movement  was  the  same  as  the 
induction  wattmeter.  There  was  a  movable  arm  and  contacts 
and  two  stationary  contacts,  one  on  each  side  of  the  movable 
contact,  with  separate  adjustments  provided  to  allow  different 
settings  for  tripping  points  in  normal  and  reverse  directions. 
The  movement  was  controlled  by  a  strong  spring  to  allow  set- 


A.C.  POWER-DIRECTIONAL  RELAYS  87 

ting  to  two  or  three  times  full  load.     No  attempt  was  made  to 
introduce  time  lag,  or  damping,  the  relays  acting  instantaneously. 

These  relays  were  found  to  be  entirely  inadequate  on  account 
of  insufficient  torque  when  short-circuits  caused  the  voltage 
and  power  factor  to  drop  to  low  values.  They  proved  con- 
clusively that  a  pure  " wattmeter"  relay  was  not  satisfactory. 

Another  early  relay  used  a  moving  coil  dynamometer-type 
movement  with  an  ironclad  magnetic  circuit  to  increase  the 
torque.  One  particular  reason  why  this  failed  was  because 
the  very  powerful  current  in  the  current  coil  generated  a  voltage 
in  the  voltage  coil  of  the  relay  during  a  short-circuit  and  conse- 
quently the  relay  would  not  trip  when  it  should.  Under  other 
conditions,  the  induced  voltage  caused  it  to  trip  when  it  should 
not. 

Another  disadvantage  of  the  instantaneous  reverse-power 
relays  was  that  sudden  momentary  surges,  such  as  might  be  due 
to  synchronizing  or  switching,  would  trip  out  the  breaker 
unnecessarily. 

There  were  many  attempts  made  to  add  corrective  features 
to  the  early  wattmeter  relays.  One  of  the  most  interesting 
was  an  arrangement  consisting  of  a  contact  device  in  com- 
bination with  a  quick-acting  regulator  to  maintain  the  current 
practically  constant  in  the  potential  coil  regardless  of  low 
voltage.  Adjustments  were  provided  to  vary  the  time  element 
of  the  relay. 

The  next  step  in  development  was  the  " differential"  type, 
in  which  a  voltage  coil  was  added  to  " polarize"  an  ordinary 
current-operated  relay,  in  order  to  cause  the  relay  to  operate 
at  a  lower  value  pf  current  in  the  reverse  than  in  the  normal 
direction.  Such  relays  were  made  in  both  the  solenoid-bellows 
type  and  the  induction  type.  In  the  solenoid  type,  a  voltage 
winding  was  superimposed  upon  the  current  solenoid:  voltage 
and  current  acting  in  opposition  on  normal  flow,  and  acting 
additively  if  the  power  flow  reversed. 

Another  form  of  this  differential  class  of  relay  was  made 
on  the  induction-type  wattmeter  principle,  by  winding  the 
relay  for  excess  current  and  adding  a  voltage  winding  connected 
to  a  voltage  transformer. 

The  coils  were  wound  on  the  iron  laminations  in  such  relation 


88  PROTECTIVE  RELAYS 

as  to  cause  the  torque  in  the  movable  disk  to  be  proportional 
to  the  square  of  the  current;  a  terminal  block  was  used  to  vary 
the  current  settings  by  changing  the  number  of  turns  in  the 
main  coil.  When  voltage  was  applied  to  the  terminals  of  the 
main  coil,  the  effect  was  to  shift  the  torque  curve  in  the  reverse 
direction.  It  will  be  understood  that  the  great  advantage 
of  this  .type  of  relay  over  the  earlier  wattmeter  types  was  that 
even  should  the  voltage  or  the  power  factor,  or  both,  fall  to 
zero,  the  relay  would  become  a  plain  "  excess-current "  relay, 
and  thus  trip  out  the  circuit  breaker.  Whereas  the  wattmeter 
relays  would  be  inactive  under  these  conditions,  resulting  in 
no  automatic  protection,  and  these  relays  would  trip.  Their 
difficulty,  was,  of  course,  that  under  such  conditions  they  could 
not  discriminate  between  directions  of  power  flow  and  thus 
would  trip  out  both  circuit  breakers  at  the  substation  ends 
of  parallel  lines.  But  they  were  better  than  relays  which  under 
conditions  of  low  voltage  or  power  factor  would  not  trip  out 
at  all,  and  they  were  thus  used  satisfactorily  for  a  number  of  years. 

These  relays  were  also  made  for  polyphase  work  with  two 
movements  operating  one  shaft  and  contact,  but  as  the  tripping 
values  would  be  different,  according  to  whether  the  overload 
was  on  one  phase  or  on  more,  they  were  abandoned  in  favor 
of  the  use  of  separate  single-phase  elements  for  polyphase  service. 

As  regards  their  place  in  the  field,  these  "  overload  and  reverse  " 
relays  can  only  be  regarded  as  a  modification  of  "overload'7 
(excess-current)  relays,  which,  with  given  conditions  of  super- 
imposed voltages  as  regards  value  and  direction,  will  trip  at 
different  values  of  current. 

They  become  practically  " current"  relays  when  the  voltage 
drops  very  low. 

REQUIREMENTS  OF  A  PRACTICAL  POWER-DIRECTIONAL  RELAY 

From  the  foregoing  it  is  evident  that  a  practical  power- 
directional  relay  should  fulfill  the  following  conditions: 

1.  It  should  close  its  contacts  positively  when  the  direction 
of    power    flow    is    reversed,  under  all  possible  conditions    of 
voltage,  power  factor  and  current. 

2.  It  should  never,  under  any  circumstances,  close  its  contacts 
when  the  direction  of  power  flow  is  normal. 


A.C.  POWER-DIRECTIONAL  RELAYS  89 

3.  The  excess  current  and  the  directional  element  must  be 
mechanically  separate  so  that  the  directional  element  has 
no  influence  on  the  characteristics  of  the  excess-current  element, 
but  merely  determines  if  this  excess  current  is  in  the  " normal" 
or  "  re  verse"  direction. 

These  are  the  fundamental  requirements.  In  addition  it 
is  desirable  to  have  a  time  element  which  can  be  accurately 
predetermined  and  quickly  adjusted  to  any  desired  value. 

It  is  assumed,  as  a  matter  of  course,  that  the  relays 
are  reliable  and  rugged  in  their  mechanism,  and  have  the 
necessary  current-carrying  capacity  both  in  windings  and 
contacts.  These  two  requirements  immediately  eliminate  all 
relays  having  mutual  inductance  between  the  current  and 
voltage  coils  such  as  the  dynamometer  type  has  at  present. 

A  most  important  step  toward  fulfilling  these  conditions  was 
in  adding  a  separate  wattmeter  element  with  contacts  in  series 
with  those  of  the  excess-current  relay.  The  wattmeter  element 
was  very  carefully  constructed,  with  a  weak  spring  and  quick 
time  element,  so  that  the  least  flow  'of  current  in  the  reverse 
direction  would  close  the  contacts  and  thus  allow  the  excess- 
current  relay  to  trip  the  breaker,  in  the  event  of  excess  current 
in  the  reverse  direction.  With  current  in  the  normal  direction, 
the  wattmeter  contacts  remained  open,  so  that  even  should  an 
excess  current  cause  the  excess-current  relay  contacts  to  close,  it 
still  could  not  trip  the  breaker,  because  the  wattmeter  contacts 
were  in  series  with  excess-current  relay  contacts  and  the  breaker 
could  not  trip  until  both  contacts  closed:  i.e.  an  excess  current 
in  the  reverse  direction  only  and  never  in  the  normal  direction. 
This  combination  clearly  selected  between  an  overload  in  the 
normal  and  reverse  direction  even  if  the  voltage  dropped  to  -2 
per  cent  of  normal  and  the  power  factor  to  10  per  cent. 

LATEST  DEVELOPMENTS 

In  Fig.  96  is  shown  one  of  the  most  highly  developed  types  of 
power-directional  relay  on  the  market  today.  This  relay  combines 
all  the  points  heretofore  mentioned  and  has  so  far  met  the 
most  exacting  conditions  of  parallel  feeders,  ring  systems  and 
networks  of  the  heaviest  and  most  intricate  power  systems  of 
the  country.  It  will  be  noticed  that  the  single  case  con- 


90 


PROTECTIVE  RELAYS 


tains  two  separate  induction  elements,  each  with  its  windings, 
disk,  magnets,  contacts,  etc.  There  is  no  mechanical  connec- 
tion whatever  between  the  two  moving  elements.  The  top 
element  is  the  quick-acting  extremely  sensitive  watt  element 
and  the  lower  element  is  the  standard  "  excess-current "  (over- 


FIG.  96a.  FIG.  966. 

FIG.  96a.— External  view  of  Westinghouse  power  directional  relay. 
FIG.  966. — Internal  view  of  Westinghouse  power  directional  relay. 

load)  relay,  provided  with  adjustable  time  lever,  etc.  exactly 
as  described  for  the  overload  inverse-definite-minimum  time- 
limit  relay. 

Figure  97  shows  the  internal  wiring  diagram  of  connections, 
the  current  winding  being  shown  by  the  heavy  black  lines. 
Figure  98  shows  the  same  connections  but  the  voltage  circuit 
is  this  time  shown  by  heavy  lines,  and  Fig.  99  shows  the  same, 
with  the  trip  circuit  shown  heavy. 

The  Contactor  Switch. — Another  effect  present  during  short- 
circuit  was  that,  due  to  the  flow  of  heavy  currents,  the  vibration 
of  the  disks  prevented  the  making  of  good  contacts  at  the  watt 
element  and  also  caused  the  disks  to  slip  on  the  shafts  due  to  the 


A.C.  POWER-DIRECTIONAL  RELAYS 

,TOP  CONTACTS 


91 


Jj 
*rV 


— —  _       

t       Fiu.  97.— IMel'llai  diagram 'of  Wy«tlllghouse  power  directional  relay  with  seri( 
circuit  shown  with  heavy  lines. 


FIG.  98. — Same  as  Fig.  97  except  with  potential  circuit  shown  heavy. 


FIG.  99. — Same  as  Fig.  97  except  with  trip  circuit  shown  heavy. 


92  PROTECTIVE  RELAYS 

excessive  torque  developed.  The  vibration  effect  was  a  matter 
of  magnetic  attraction  between  the  series  coils  and  the  induced 
currents  in  the  disk. 

Fortunately,  this  weak,  fluttering  contact  may  be  changed 
into  a  good  positive  contact  by  means  of  a  contactor  switch, 
mounted  in  the  case.  This  consists  of  a  solenoid,  capable  of 
attracting  an  iron  plunger  which  carries  a  silver  contact,  as  in  Fig. 
84. 

Now  suppose  the  main-relay  contacts  just  barely  close  and  are 
vibrating  or  chattering  violently  due  to  the  heavy,  induced 
currents  in  the  disk  to  which  they  are  attached.  Their  partial 
touching  allows  a  weak,  fluttering  current  to  flow  in  the  solenoid. 
This  weak  current  immediately  raises  the  plunger,  and  the 
contact  disk  immediately  short-circuits  the  two  stationary 
contacts,  which,  being  in  parallel  with  the  main  contacts,  immedi- 
ately take  all  the  trip  current  by  making  good  positive  contact, 
thus  positively  energizing  the  trip  coil  and  tripping  the  breakers. 

It  is  very  evident  that  should  the  main  contacts  still  flutter, 
there  will  be  no  spark,  and  in  fact,  after  the  first  touch  they 
may  even  open,  although  the  touch  was  enough  to  close 
the  contactor  contacts  and  trip  the  breaker.  The  trip  circuit 
will  stay  energized,  and  these  contacts  will  stay  closed  until 
the  circuit  is  opened  by  the  auxiliary-pallet  switch  on  the  breaker, 
which  is  arranged  for  this  purpose. 

The  contactor  switch  not  only  assures  positive  contact, 
but  by  increasing  the  tripping-circuit  capacity  from  2  to  20 
amp.,  avoids  the  use  of  an  auxiliary-relay  switch,  except  in  the 
case  of  very  heavy  trip  currents. 

The  Torque  Compensator. — The  effect  of  the  excessive 
torque  loosening  the  disks  on  the  shafts  was  easily  overcome 
by  means  of  the  "torque  compensator, "  which,  as  was  thoroughly 
explained  under  "Induction-type  Relays,"  is  merely  a  small  trans- 
former with  the  primary  carrying  the  main  current,  the  secondary 
supplying  the  relay  windings,  and  so  proportioned  that  the 
iron  becomes  saturated  at  heavy  overloads,  thus  preventing 
excessive  flow  of  current  in  the  relay. 

Stray  Fields. — In  view  of  the  fact  that  the  heavy  overload 
currents  produce  such  a  strong  field,  it  might  be  suspected 
that  this  alternating  field  would  weaken  the  permanent  magnets 


A.C.  POWER-DIRECTIONAL  RELAYS 


93 


and  change  the  time.  This,  however,  is  not  the  case,  as  the  mag- 
nets are  placed  on  the  diametrically  opposite  side  of  the  disk  from 
the  driving  coils,  and  thoroughly  shielded  by  the  framework 
exactly  as  in  a  watthour  meter. 

Moreover,  in  testing,  the  relays  are  subjected  to  many  times 
more  current  than  they  would  ever  get  in  actual  practice, 
so  any  demagnetizing  effect  would  make  a  permanent  change 
the  first  time  the  relay  was  tested  and  any  following  excess 
current  could  not  possibly  damage  its  action. 


FIG.  100.— G.  E.  power  directional  relay. 


Other  Relays. — Instead  of  combining  a  watt  element  and  an 
overload  relay  in  one  case,  and  using  three  combination  instru- 
ments on  each  three-phase  line,  some  engineers  prefer  to  house 
the  three  watt  elements  in  one  case  and  use  this  relay  in  con- 
junction with  two  or  three  overload  relays,  either  plunger 
or  induction  type.  Such  a  relay  is  shown  in  Figs.  100  and  101. 
In  Fig.  102  is  shown  the  internal  wiring  diagram. 


94 


PROTECTIVE  RELAYS 


FIG.  101. — Interior  view  of  G.  E.  polyphase  power  directional  relay. 


Tr/pping  Circuit  • 


o  -  ^S&S&S&J  -  \ 


Coils  of  Uppers 
Element 


Coils  or  Lower  >\ 
ffear  E/emenj  • 


Coils  of  Lower', 
front  E/emenf> 


FIG.  102. — Internal  wiring  diagram  of  G.  E.  polyphase  power  directional  relay. 


A.C.  POWER-DIRECTIONAL  RELAYS  95 

From. this  it  will  be  plainly  seen  that  there  are  three  sepa- 
rate current  coils  and  three  separate  potential  coils,  and  since 
the  relay  operates  on  the  induction  principle,  there  is  no  mutual 
inductance  between  coils;  consequently  no  induced  voltage 
in  the  potential  coil  from  the  heavy  current  flowing  in  the 
series  coil. 

Two  disks  are  used,  the  upper  one  of  which  is  driven  by  one 
element.  The  lower  disk  is  driven  by  two  elements,  one  in  the 
front  (plainly  shown  in  Fig.  101)  and  one  in  the  rear.  Under 
normal  direction  of  power,  the  disks  tend  to  rotate  in  one  direction 
and  keep  the  contact  open.  Reversal  of  power  flow  causes  the 
disks  to  rotate  in  the  opposite  direction,  thus  closing  the  contact. 
Since  the  action  desired  is  as  nearly  instantaneous  as  possible, 
no  attempt  is  made  to  damp  the  movement;  the  arc  of  disk 
travel  is  very  small. 

Fluttering  trip  currents  such  as  might  be  due  to  vibration 
of  disk,  bouncing  of  contacts,  etc.  are  quickly  transformed 
into  full-strength  current  by  means  of  the  small  auxiliary  con- 
tactor switch. 

Power-directional  Multi-element  Relays  Used  with  Overload 
Relays. — This  power-directional  relay  must  be  used  in  conjunc- 
tion with  a  three-phase  or  three  single-phase  overload  relays  such 
as  the  induction  type  or  solenoid-bellows  type,  to  secure  proper 
protection. 

The  trip  circuits  of  the  overload  relays  (connected  in  mul- 
tiple) are  connected  in  series  with  that  of  the  power-directional 
relay,  so  that  neither  can  trip  the  breaker  separately,  but 
both  must  close  their  contacts  before  the  breaker  will  be  tripped. 

Each  overload  relay  is  connected  in  series  with  a  current  coil 
of  the  reverse-power  relay.  The  complete  connections  of  a  three- 
phase  circuit  are  shown  in  Figs.  159  and  160.  This  plainly 
shows  the  main  three-phase  line,  protected  by  breaker,  and  sup- 
plied with  three  current  and  three  voltage  transformers.  Each 
current  transformer  supplies  one  current  coil  in  the  power  direc- 
tional relay  and  one  overload  relay.  The  trip  of  the  power-direc- 
tional relay  is  connected  in  series  with  the  trips  of  the  overload 
relays  and  operates  the  trip  coil  of  the  breaker.  When  the  break- 
er opens,  the  auxiliary  switch  opens,  thus  resetting  the  contactor 
switch  in  the  power-directional  relay. 


96 


PROTECTIVE  RELAYS 


Differential  Power-directional  Relay. — Another  recent  devel- 
opment is  the  differential  power-directional  relay.  This  must 
be  used  in  conjunction  with  the  auxiliary  relay  shown  in  Fig.  103. 

This  relay  is  also  shown  with  the  cover  removed  in  Fig.  103 
and  the  diagram  of  connections  is  shown  in  Fig.  164.  The  power- 
directional  relay  is  arranged  to  make  contact  when  it  moves  either 
to  the  left  or  right.  But  once  it  closes  one  contact,  it  instantly 
energizes  one  of  the  interlocking  relays  and  this  relay  opens  the 


FIG.  103. — Westinghouse  auxiliary  interlocking  relays  with  and  without 
protecting  cover. 

trip  circuit  to  the  opposite  contact.  The  interlocking  relay 
resets  in  about  two  seconds  after  the  trip  circuit  has  been  de-ener- 
gized by  the  opening  of  the  circuit  breaker. 

The  applications  of  power-directional  relays  to  various  systems 
will  be  discussed  in  a  subsequent  chapter  devoted  to  that  subject. 

Specifications. — In  order  that  engineers  may  obtain 
the  highest  grade  of  power-directional  relays  and  to  guard 
against  the  use  of  obsolete  and  defective-principled  relays 


A.C.  POWER-DIRECTIONAL  RELAYS  97 

it  would  be  well  for  engineers  and  architects  to  include  the 
following  in  their  specifications:  " Power-directional  relays 
should  consist  of  two  separate  and  complete  elements,  one  which 
operates  on  excess  current,  as  described  hereafter,  and  one  which 
operates  on  a  reversal  of  power  as  described  hereafter.  These 
elements  may  or  may  not  be  mounted  in  the  same  housing. 
The  current  element  shall  be  equipped  with  a  time  delay  that 
varies  inversely  with  the  current  at  all  moderate  overload 
and  which  becomes  definite  at  heavy  overloads.  This  definite 
time  shall  be  adjustable  for  all  values  between  0  and  2  sec. 
(or  0  and  4  sec.)  which  adjustment  shall  be  accurate  and  per- 
manent. The  relay  shall  be  equipped  with  a  convenient 
terminal  board  by  which  the  current  values  for  tripping  may  -be 
adjusted  easily  to  a  number  of  different  values,  say  4,  5,  6,  7  and 
8  amp.  for  instance. 

"The  element  which  operates  on  reversal  of  power  shall  be 
selective  with  regard  to  direction  of  power  flow  under  all  con- 
ditions of  low  power  factor  and  unbalanced  short-circuits, 
and  shall  be  selective  at  2  per  cent  of  normal  voltage  upon 
the  occurrence  of  an  overload  of  300  per  cent  or  more.  They 
shall  be  so  constructed  that  the  normal  power,  including  moderate 
overloads,  can  flow  in  either  direction  through  the  circuit 
to  which  the  relays  are  connected,  without  causing  the  relays 
to  trip  the  circuit  breaker." 

This  specification  of  course,  is  in  addition  to  the  regular 
specifications  covering  workmanship,  material,  etc. 


CHAPTER    IX 

CHARACTERISTICS  OF  A.C.  DISTURBANCES 

Many  of  the  early  protective  relays  were  designed  with 
but  a  partial  knowledge  of  the  actual  characteristics  of  the 
electrical  disturbances  which  they  were  supposed  to  detect 
and  isolate.  Consequently,  there  were  many  failures  under 
certain  conditions,  and  these  failures  led  to  an  intimate  study 
of  the  effects  of  an  electrical  disturbance  from  both  a  theoret- 
ical and  an  actual  standpoint.  In  some  instances,  the  lines 
were  actually  shorted  at  various  locations  to  determine  the 
actual  conditions,  while  in  other  cases,  miniature  systems  were 
built,  with  lines  having  characteristics  similar  to  the  main 
line,  in  order  to  study  the  extent  and  divisions  of  overloads. 

Even  the  protection  of  simple  apparatus  such  as  motors  and 
transformers  requires  an  intimate  knowledge  of  how  certain 
apparatus  acts  in  ease  of  electrical  distress. 

Some  of  the  more  important  points  on  which  an  accurate 
knowledge  must  be  obtained  are  as  follows: 

1.  What  is  the  intensity  of  a  " short-circuit"? 

2.  For  how  long  can  the  overload  exist? 

3.  What  is  the  effect  on  the  system  voltage? 

4.  What  other  effects,  such  as  phase  distortion  and  surges, 
accompany  severe  disturbances? 

5.  What  must  the  relay  do  and  what  must  it  not  do,  and 
what  are  the  best  connections? 

These  points  are  essential,  not  only  to  the  user  of  the  pro- 
tective relays,  but  also  to  the  manufacturer,  and  it  is  safe  to  say 
that  had  the  manufacturers  had  proper  information  on  these 
points  when  they  designed  their  first  relays,  the  relay  user 
could'  have  saved  many  thousands  of  dollars  of  damage  to 
apparatus  and  avoided  thousands  of  interruptions. 

EFFECTS  OF  OVERLOAD 

The  duration  of  an  overload  without  damage  depends  entirely 
on  the  apparatus  itself;  that  is,  it  may  be  sustained  until  the 
excessive  heat  starts  to  burn  the  insulation,  or  cause  other 

98 


CHARACTERISTICS  OF  A.C.  DISTURBANCES  99 

effects  which  would  lead  to  damaged,  insulation.  Damage 
from  overload  is  seldom  caused  except  by  heat,  some  exceptions 
being  the  breaking  of  shafts  or  the  explosion  of  transformers, 
or  the  puncturing  of  insulation  due  to  surges.  Modern  design 
has,  however,  practically  eliminated  this  danger. 

Some  motors  may  often  have  their  voltage  reduced  to  zero 
for  a  second  or  so  without  being  damaged  or  losing  their  load; 
they  also  may  carry  overload  (about  50  per  cent)  continuously 
without  serious  damage;  but  they  must  be  protected  so  that 
they  will  be  cut  out  of  service  should  the  load  exceed  150  per  cent 
for  any  length  of  time.  If  internal  short-circuits  develop, 
causing  excess  current,  they  must  be  cut  out  very  quickly  to 
avoid  serious  burnouts. 

Power  transformers  must  also  be  protected  against  internal 
and  external  overloads  in  the  same  manner;  in  fact,  an  internal 
short  in  a  large  transformer  might  damage  it  severely  in  a 
few  seconds,  and  as  an  internal  short  will  seldom  clear  itself, 
its  automatic  isolation  should  be  practically  instantaneous. 

Transmission  lines  are  often  damaged  by  prolonged  overload, 
and  they  also  cause  an  excess  current  in  the  generators  to  which 
they  are  connected  in  case  of  short-circuits. 

One  of  the  big  present-day  problems  in  automatic  sectional- 
izing  is  to  cut  out  a  short-circuited  section  of  line  before  it  burns 
down. 

The  overload  which  an  alternator  can  stand  depends  entirely 
upon  the  alternator,  as  the  characteristics  of  such  machines  vary 
over  a  wide  range.  The  short-circuit  current  may  be  roughly 
calculated  by  observing  the  voltage  drop  between  the  two 
sections  at  normal  load,  as  will  be  explained  later. 

The  current  during  a  short-circuit  decreases  very  rapidly 
until  a  sustained  short-circuit  current  is  reached,  as  will  be  seen 
by  referring  to  Fig.  104.  This  curve  does  not  mean  that  the 
maximum  possible  short-circuit  current  is  100  per  cent,  but 
take  as  example  an  alternator  which  gives  12  times  the  normal 
load  current  on  short-circuit.  This  is  100  per  cent,  but  this 
12  times  quickly  decreases  until  the  sustained  current  is  about 
12  per  cent  of  12  times  or  1.4  times  the  normal  load. 

Some  alternators  may  deliver  a  sustained  short-circuit  cur- 
rent of  two  and  one-half  or  three  times  the  full-load  current. 


100 


PROTECTIVE  RELAYS 


This  rapid  decrease  of  short-circuit  current  is  another  important 
reason  why  a  breaker  should  not  trip  out  instantly.  Its  breaking 
capacity  must  be  considerably  greater  to  trip  instantly  than 
to  trip  when  the  current  falls  to  its  sustained  value,  which  is  much 
lower. 


FIG.    104. — Current  decrease  on  asymmetrical  short-circuits. 

NATURE  OF  SHORT-CIRCUITS  ON  TRANSMISSION  LINES 

When  making  current  calculations  it  should  always  be  assumed 
that  a  short-circuit  is  due  to  a  metallic  connection  between  the 
conductors.  On  a  high-voltage  aerial  line  using  wooden  pins 
and  crossarms,  it  sometimes  happens  that  an  insulator  is 
broken,  with  the  result  that  the  wood  is  gradually  heated 
by  the  passage  of  the  current  through  it  until  it  finally  bursts 
into  flame,  thus  causing  an  arc  between  conductors.  A  little 
consideration  shows  that  the  flow  of  the  current  is  small  until 
the  arc  is  established,  and  that  it  is  absurd  to  speak  of  auto- 
matically disconnecting  a  section  of  line  which  has  such  a  high- 
resistance  short-circuit.  It  has  sometimes  been  assumed  that 


CHARACTERISTICS  OF  A.C.  DISTURBANCES  101 

an  arc  has  a  high  resistance,  but  this  iy  not:  the  case/  and  in 
general  the  presence  of  an  arc  at  .the  point -of  s/ioii-mv.iit 
will  not  decrease  the  short-circuit  curre-nt  by  more  ohan  a  few 
per  cent.  Incidentally,  it  may  be  of  interest  to  note  that  on 
a  high-voltage  ungrounded-neutral  system  the  capacity  current 
to  ground  through  an  arc  may  be  greater  than  it  is  through  a 
direct  ground.  There  is  only  one  case  where  a  short-circuit  is 
likely  to  increase  in  intensity  as  it  develops,  and  that  is  on 
a  system  where  the  neutral  is  grounded  through  a  resistance; 
a  cable  breakdown,  for  instance,  frequently  occurs  first  between 
one  conductor  and  the  sheath  and  the  current  flow  may  be 
limited  by  the  neutral  resistance ;  the  trouble  will  quickly  involve 
all  the  conductors  in  the  cable,  resulting  in  a  heavy  short-circuit, 
but  it  is  possible  that  it  will  require  an  appreciable  time  to 
do  this,  in  which  case  the  relay  operation  may  be  unsatisfactory. 
This  is  particularly  liable  to  happen  if  the  neutral  is  not  grounded 
at  every  substation. 

CALCULATION  OF  THE  SHORT-CIRCUIT  CURRENT 

In  applying  any  protection  scheme,  it  is  necessary  to  determine 
the  short-circuit  currents  which  may  develop  under  all  condi- 
tions. It  is  unfortunate  that  the  term  "  overload"  has  come 
into  use  in  connection  with  sectionalizing  distribution  systems, 
because  it  implies  that  the  relays  should  be  set  to  operate  at 
a  value  determined  by  the  normal  load  on  the  feeder. 

Such  a  setting  is  possible  if  definite-time-limit  relays  are  used, 
but  where  a  relay  having  inverse-time  characteristics  is  used 
it  is  necessary  to  consider  the  current  that  occurs  during  times 
of  trouble,  and  that  maybe  tens  or  even  hundreds  of  times  greater 
than  the  normal  current.  An  approximate  method  of  deter- 
mining the  possible  short-circuit  current  is  by  observing  the 
voltage  drop  between  two  stations  at  normal  load.  Short- 
normal  voltage  .  , 

circuit  current    = rr—  — - r —       X  load  current. 

voltage  drop 

For  example,  if  a  certain  load  current  causes  a  drop  of  5 
per  cent  in  voltage  between  a  generator  station  and  substation, 
the  maximum  short-circuit  current  would  be  20  times  the  load 
current.  Results  obtained  in  this  way  are  likely  to  be  too  large, 
particularly  on  lines  having  high  inductance. 


102 


PROTECTIVE  RELAYS 


The  calculation  of \ the  short-circuit  currents  on  a  compli- 
cated :&ysttim  involves  more  or  less  approximation,  and  a  good 
method  is'to  prepare  a  table  showing  the  impedance  of  each 
section  of  line  and  also  of  the  generators.  These  figures  can  then 

FIG.  105. — RESISTANCE,  INDUCTANCE  AND  IMPEDANCE  OF  OVERHEAD  LINES 
Resistance  (R)  \         Inductance  X  and  impedance  Z  per  wire  per  mile 


Spacing,  ft. 

2 

4 

8 

15 

Size  wire 

R 

X 

Z 

•yr 

Z 

X 

^ 

X 

Z 

25  Cycles 


0000 

0.267 

0.245 

0.365 

0.280 

0.387 

0.315 

0.413 

0.348 

0.437 

000 

0.336 

0.251 

0.420 

0.286 

0.442 

0.320 

0.463 

0.352 

0.487 

00 

0.423 

0.257 

0.495 

0.291 

0.563 

0.326 

0.535 

0.358 

0.553 

0 

0.534 

0.262 

0.595 

0.297 

0.611 

0.332 

0.628 

0.364 

0.647 

2 

0.849 

0.277 

0.895 

0.312 

0.905 

0.347 

0.917 

0.378 

0.930 

4 

1.35 

0.288 

1.38 

0.324 

1.39 

0.358 

1.396 

0.390 

1.40 

6 

2.15 

0.401 

2.19 

8 

3  400 

0  413 

3  43 

60  Cycles 


0000 

0.267 

0.587 

0.645 

0.672 

0.723 

0.755 

0.801 

0.831 

0.873 

000 

0.336 

0.601 

0.690 

0.685 

0.763 

0.769 

0.839 

0.845 

0.908 

00 

0.423 

0.615 

0.745 

0.699 

0.815 

0.782 

0.888 

0.859 

0.958 

0 

0.534 

0.629 

0.825 

0.714 

0.892 

0.797 

0.958 

0.873 

1.03 

2 

0.849 

0.664 

1.075 

0.748 

1.130 

0.832 

1.188 

0.908 

1.23 

4 

1.35 

0.692 

1.515 

0.776 

1.555 

0.860 

1.60 

0.936 

1.64 

6 

2  15 

0.964 

2.35 

8 

3.40 

0.992 

3.54 

Above  values  are  to  be  used  with  voltage  to  neutral.  Sizes  No.  0000  to  0 
are  stranded:  others  are  solid.  Based  on  97  per  cent  conductivity  at  20°C. 
or  67°F.  Values  in  table  computed  on  slide  rule. 

be  combined  in  any  way  desired  to  determine  the  impedance  of  a 
particular  path.  In  obtaining  the  impedance  of  several  sections 
of  a  system,  the  resistances  and  inductances  must  be  added 
separately  and  the  two  sums  combined  geometrically.  The 
inductance  varies  with  the  size  of  the  conductors  and  with  the 


CHARACTERISTICS  OF  A.C.  DISTURBANCES 


103 


distance  between  them,  which  in  the  case  of  a  cable  is  determined 
by  the  thickness  of  the  insulation.  The  characteristics  of 
cable  can  usually  be  obtained  from  the  manufacturers.  A 
15,000-v.  No.  0000  cable  at  60  cycles  has  an  impedance  about  23 
per  cent  greater  than  its  ohmic  resistance,  whereas,  the  imped- 
ance of  a  150,000-v.  line  having  the  same  size  copper  conductor 
spaced  15  ft.  apart  is  about  three  and  one-quarter  times  the 

FIG.  106. — APPROXIMATE  OHMIC  RESISTANCE  AND  IMPEDANCE  OF  THREE- 
CONDUCTOR  CABLES,  AT  60  CYCLES 


Resist- 

Impedance, ohms  per  mile 

Size 

ance, 

Working  voltage 

ohms 

per  mile 

3,000 

5,000 

7,000 

10,000 

is.  ,000 

20,000 

2 

0.850 

0.858 

0.859 

0.863 

0.867 

0.872 

0.884 

1 

0.674 

0.692 

0.696 

0.700 

0.706 

0.712 

0.724 

0 

0.535 

0.545 

0.547 

0.552 

0.558 

0.565 

0.580 

00 

0.424 

0.436 

0.439 

0.444 

0.452 

0.460 

0.478 

000 

0.336 

0.352 

0.352 

0.357 

0.365 

0.374 

0.396 

0000 

0.267 

0.280 

0.283 

0.288 

0.296 

0.306 

0.332 

250000 

0.227 

0.245 

0.245 

0.252 

0.261 

0.272 

0.299 

300000 

0.188 

0.210 

0.210 

0.217 

0.227 

0.241 

0.270 

350000 

0.161 

0.187 

0.187 

0.194 

0.204 

0.217 

0.250 

400000 

0.141 

0.166 

0.166 

0.174 

0.185 

0.199 

0.234 

450000 

0.127 

0.148 

0.148 

0.156 

0.167 

0.182 

0.221 

500000 

0.113 

0.137 

0.137 

0.144 

0.156 

0.172 

0.212 

Based  on  Pure  Copper,  75°F.  with  an  allowance  of  3  per  cent  for  spiral 
path  of  conductors,  60  cycles  per  second  and  standard  thickness  of  varnished 
cambric  insulation.  Values  are  practically  the  same  for  other  types  of 
insulation.  These  figures  are  also  approximately  correct  for  98  per  cent 
conductivity  copper  at  65°F. 


value  of  its  resistance.  The  resistance,  inductance  and  impedance 
of  aerial  transmission  lines  having  various  wire  spacing  are  given 
in  Fig.  105,  and  Fig.  106,  which  shows  the  resistance  and  impe- 
dance of  various  kinds  of  three-conductor  cable. 

The  method  of  computing  the  impedance  of  a  circuit,  including 
a  line,  generator  and  transformer,  is  shown  in  the  following 
example : 


104  PROTECTIVE  RELAYS 

Assume. — A  5,000-kva.,  60-cycle  generator  having  10  per  cent  reactance 

drop. 
A  5,000-kva.  bank  of  transformers  having  1  per  cent  resistance 

drop  and  5  per  cent  reactance  drop. 

50  miles  45,000-v.  line  No.  0  copper  conductors  spaced  4  ft. 
apart. 

All  values  of  resistance,  reactance  and  impedance  will  be  reduced  to  terms 
of  45,000  v. 

Full-load  current  =      ,-_.  =  =  64  amp. 

V3  X  45,000 

Star  voltage  =  26,100. 
Generator  Characteristics: 

Reactance  drop  =  10  per  cent  of  26,100  =  2,610  v. 

Reactance  =    '         =41  ohms. 

Transformer  Characteristics : 

Resistance  drop  =  1  per  cent  of  26,100  =  261  v. 

OA1 

Resistance  =  -TCT  =4.1  ohms. 
64 

Reactance  drop  =  5  per  cent  of  26,100  =  1,305  v. 

Reactance  =  -htj-  =  20  ohms. 
64 

Line  Characteristics  (from  Table,  Fig.  105) : 

R  =  50  X  0.534  =  26.7  X  =  50  X  0.714  =  35.7 

Summary: 

R  X 

Generator Negligible  41 . 0 

Transformer 4.1  20 . 0 

Line..                                              26.7  35.7 


Total 30.8  96. 7  ohms. 

R2  =          950 
X2  =       9,150 
Z2  =  R2  +  X2  =  10,100  Hence  Z  =  100.5  ohms 

2fi  1 00 
The   short-circuit    current  is   therefore    ^  5    =    260   amp. 

for  the  first  instant.  As  shown  in  Fig.  94,  tne  initial  current 
will  decrease  until  the  sustained  value  is  reached.  In  this 
example  the  sustained  value  is  probably  about  twice  full-load 
current,  or  say  130  amp.  If  the  lines  should  have  more  impe- 
dance, or  if  less  generating  capacity  should  be  connected  to 
the  busbars,  the  generator  reaction  would  have  less  effect 


CHARACTERISTICS  OF  A.C.  DISTURBANCES  105 

in  cutting  down  the  current,  and  the  calculated  results  would 
need  less  correction. 

Alternator  and  Transformer  Constants. — The  characteristics 
of  alternators  vary  through  a  wide  range,  but  it  is  usually 
assumed  that  their  reactance  is  about  8  per  cent,  which  allows 
a  maximum  instantaneous  root-mean-square  asymmetrical 
short-circuit  current  of  19  times  full  load.  The  maximum 
sustained  short-circuit  current  is  usually  assumed  to  be  between 
two  and  one-half  and  three  times  full  load,  although  some 
machines,  particularly  turbo-alternators,  are  now  being  built 
which  have  a  sustained  short-circuit  current  of  about  one  and 
one-half  times  full  load.  It  is  usually  safe  to  assume  that  a 
transformer  has  1  per  cent  resistance  drop  and  from  3  to  4  per 
cent  reactance  drop. 

Effect  of  Low  Voltage. — The  most  important  requirement 
of  a  power-directional  relay  is  that  it  will  operate  when  the  po- 
tential at  its  terminals  is  between  1  and  2  per  cent  of  normal. 
If  we  assume  the  case  of  a  No.  0000  cable  normally  carrying 
300  amp.  at  12,000  v.,  connected  to  a  generating  station  having 
a  short-circuit  current  of  3,000  amp.,  the  loss  which  would  occur 
between  the  busbars  and  a  metallic  short-circuit  100  ft.  from 
them  would  be  45  kw.  per  phase,  or  less  than  three-quarters  of 
1  per  cent  of  the  relay  setting.  This  shows  the  absurdity 
of  installing  relays  which  require  a  percentage  reversal  of  5 
or  10  per  cent  to  operate  them. 

The  statement  has  frequently  been  made  that  a  power- 
directional  relay  cannot  operate  when  there  is  no  voltage,  but 
neither  can  there  be  a  flow  or  current  unless  there  is  a  difference 
of  potential.  The  problem  is  therefore  nothing  more  than  a 
question  of  securing  a  contact-making  wattmeter  which  is 
sensitive  enough  to  operate  on  the  small  potential  that  is  always 
present  when  a  short-circuit  occurs.  Numerous  tests  have 
been  made  which  show  that  when  a  cable  breaks  down  the 
arc  through  the  insulating  space  between  conductors  will  main- 
tain a  voltage  of  between  1  and  2  per  cent,  and  it  has  been  found 
that  a  higher  voltage  is  maintained  when  the  current  is 
small  than  when  it  is  excessive,  a  fact  which  materially  assists 
power-directional  relays.  It  should  be  pointed  out  that  on 
large  systems  it  is  practically  impossible  to  obtain  a  metallic 


106  PROTECTIVE  RELAYS 

short-circuit  because  any  small  object  that  could  be  brought 
into  contact  with  the  busbars  would  be  immediately  destroyed. 
The  only  possibility  for  obtaining  a  short-circuit  that  will  lower 
the  voltage  to  a  point  where  reverse-power  relays  cannot  operate 
is  the  case  of  an  extra-high-voltage  system  where  the  short- 
circuit  current  is  so  small  that  it  cannot  burn  off  a  metallic 
connection.  For  instance,  on  a  150,000-v.  system  of  some 
magnitude,  the  current  at  short-circuit  may  not  exceed  500 
amp.,  which  could  be  carried  for  some  seconds  by  a  telephone 
wire  dropped  across  a  transmission  line.  The  possibility 
of  interruption  from  this  cause  is  remote,  because  a  short-circuit 
across  three  wires  will  not  often  occur,  and  when  only  two 
wires  are  involved  the  low-voltage  condition  does  not  exist 
except  on  one  phase. 

Effect  of  Unbalanced  Short-circuits. — In  the  past  the  opera- 
tion of  power-directional  relays  has  been  somewhat  unsatis- 
factory, because  means  were  not  taken  to  insure  correct  operation 
at  times  when  the  power-factor  of  the  system  was  bad,  due  to 
unbalanced  short-circuits.  As  a  result  of  several  years'  investi- 
gation, it  has  been  found  the  the  methods  of  connecting  reverse- 
power  relays  with  their  potential  coils  in  star,  as  has  been  the 
usual  custom,  is  theoretically  incorrect,  and  the  relays  may 
fail  to  operate  upon  the  occurrence  of  the  most  common  form  of 
short-circuit.  When  unbalanced  short-circuits  occur,  a  large 
number- of  combinations  of  circumstamces  are  possible,  but  it  has 
been  found  that  the  most  severe  condition  is  when  only  two  con- 
ductors of  a  three-phase  line  are  short-circuited,  and  if  relays 
will  operate  properly  under  this  condition  they  will  satisfy 
practically  all  the  others. 

In  Figs.  107a  and  1076  are  shown,  in  a  rather  incomplete  way, 
the  vector  relations  on  a  simple  electric  circuit  when  a  short- 
circuit  occurs  between  the  wires  B  and  C.  Figure  1076  shows 
at  a  the  voltage  triangle  at  the  generating  station  and  at  b 
the  voltage  triangle  some  distance  from  the  generating  station. 
At  c  is  represented  the  conditions  at  the  short-circuit,  and 
it  will  be  seen  that  the  long  sides  of  the  voltage  triangle  have 
closed  in  together.  It  will  also  be  observed  that  the  two-star 
voltages,  OB  and  OC,  are  in  phase.  Referring  again  to  a, 
if  the  circuit  has  no  inductance,  the  current  which  flows  into 


CHARACTERISTICS  OF  A.C.  DISTURBANCES 


107 


the  short-circuit  will  be  in  phase  with  the  voltage  BC,  as  is  shown 
by  the  vectors  IB  and  1C.  If  such  a  condition  were  possible, 
none  of  the  relays  at  the  short-circuit  could  operate,  because 
the  power  factor  is  zero.  Since,  however,  there  is  always  induc- 
tance in  the  circuit,  the  current  will  lag  somewhat,  as  shown 
by  the  vectors  I1B  and  PC.  The  result  of  this  is  to  cause  one 
of  the  relays  at  the  short-circuit  to  operate  forwards  and  the 
other  one  to  operate  backwards.  Figure  1076  shows  the  effect 
of  an  inductive  load  on  the  system.  The  short-circuit  currents 
are  represented  by  dash  vectors,  and  the  resultant  of  the  short- 
circuit  currents  and  load  currents  by  heavy  vectors.  The 


b  c 

FIG.  107a.  FIG.  1076. 

FIG.   107a. — Vector  diagram  of  current  and  voltage  with  short  circuit  when 
load  is  non-inductive. 

FIG.   1076. — Showing  the  vectors  with  inductive  load. 


general  result  of  the  load  current  on  the  system  is  to  make  less 
pronounced  the  effect  due  to  the  short-circuit,  as  will  be  observed 
upon  comparing  b  in  Figs.  107a  and  6.  In  the  former  case, 
one  of  the  relays  operates  backwards,  but  in  the  latter  case, 
both  of  them  read  properly. 

In  the  above  explanation,  the  condition  in  only  one  line 
has  been  shown,  and  the  question  might  immediately  arise  as 
to  what  difference  it  makes  whether  or  not  one  relay  operates 
backwards,  so  long  as  one  of  them  operates  to  trip  the  circuit 
breaker.  The  answer  is  that  the  same  condition  exists  in  all 
the  good  sections  of  line  parallel  to  the  trouble,  with  the  result 
that  their  circuit  breakers  will  also  be  opened.  This  difficulty 
can  easily  be  overcome  by  using  the  delta-delta  connection 
of  voltage  transformers  as  explained  under  "  Instrument  Trans- 
formers and  Groupings. "  With  this  connection,  the  current 
in  each  relay  leads  the  voltage  in  each  relay  by  30  deg.  when 
the  line  power  factor  is  100  per  cent  instead  of  the  relay  current 
and  voltage  being  in  phase  at  100  per  cent  line  power  factor. 


108  PROTECTIVE  RELAYS 

Then  even  though  the  line  power  factor  should  drop  to  almost 
zero  during  a  short-circuit,  the  current  in  any  relay  could  not 
lag  more  than  60  deg.  behind  its  voltage. 

CHARACTERISTICS  OF  RELAYS 

From  the  foregoing  discussion,  it  will  readily  be  seen  that  the 
relays  must  be  absolutely  reliable  and  dependable  and  have 
as  well  a  high  accuracy  both  initially  and  maintained.  Relays 
for  the  protection  of  overloads,  or  rather  excess  currents,  should 
have  two  distinct  adjustments:  time  and  current.  The  time 
setting  is  of  course  easily  set  according  to  the  relay  curves  to 
obtain  the  desired  delay. 

Much  misunderstanding  has  prevailed  in  the  past  regarding 
the  current  adjustment.  In  the  case  of  straight-overload  relays, 
the  relay  is  easily  selected  and  set  according  to  the  magnitude 
of  the  load  and  the  overload.  But  on  differential  protection, 
however,  many  relays  have  been  used  with  very  low-current 
windings  in  order  to  make  them  susceptible  to  slight  unbalanc- 
ings  in  current.  The  result  is  that  they  were  too  sensitive 
and  tripped  the  breaker  on  slight  surges  due  to  throwing  the 
apparatus  on  the  line,  or  line  switching  or  synchronizing. 
Attempts  were  made  to  give  these  relays  a  time  delay  with  only 
slightly  better  results.  In  such  case,  a  relay  should  be  used 
with  a  comparatively  high  overload  setting  and  the  time  adjusted 
to  instantaneous. 

On  the  power-directional  relays,  there  should  be  three  dis- 
tinct adjustments:  time,  current,  and  direction.  The  time 
and  current  adjustments  should  be  set  same  as  the  current  relay, 
while  the  directional  adjustment  must  be  so  sensitive  that  it 
will  function  on  a  very  small  reversal  of  power  flow  even  though 
the  potential  drop  to  1  or  2  per  cent  of  normal.  ^ 

Much  confusion  has  existed  regarding  the  correct  use  of  the 
power-directional  and  the  watt  relays.  For  instance,  consider 
a  plant  supplying  part  of  its  own  power  and  buying  the  rest 
from  a  nearby  station  with  provisions  to  limit  the  power  flow. 
Neither  a  power-directional  nor  a  watt  relay  alone  will  give 
the  desired  results;  both  must  be  used. 

When  considering  the  adjustment  of  the  power-directional 
relay,  it  must  be  remembered  that  it  is  for  protection  alone, 


CHARACTERISTICS  OF  A.C.  DISTURBANCES  109 

and  so  the  current  setting  must  be  high  enough  so  that  it  can 
never  trip  no  matter  how  high  the  current  may  be  under  normal 
load  on  the  system  in  either  direction.  If  the  normal  load  current 
produces  a  maximum  of  3  amp.  in  the  secondary  of  the  trans- 
former, then  the  relay  should  be  set  for  a  minimum  of  about 
4  amp.  In  many  cases,  it  has  been  found  desirable  to  set 
these  relays  as  high  as  10  to  12  amp. 

The  time  limit  required  is  determined  solely  by  its  relations 
to  the  time  delays  of  the  other  relays  on  the  system.  Usually 
the  time  limit  is  set  rather  low  as  the  power-directional  relay 
is  usually  placed  at  the  most  critical  point  on  the  system 
and  is  expected  to  operate  first  in  case  of  trouble.  Sometimes 
they  are  even  set  to  operate  instantaneously,  but  this  should 
be  done  with  great  care  as  the  breakers  may  be  popping  out 
upon  the  occurrence  of  the  least  surge  due  to  switching  or 
synchronizing. 

As  previously  stated,  the  time  limit  is  practically  independent 
of  the  current  setting  of  the  relay  so  that  in  cases  where  instan- 
taneous action  is  desired,  nothing  is  gained  by  making  the  current 
setting  small. 

In  order  that  it  may  operate  under  the  most  severe  short- 
circuit  conditions  where  the  voltage  is  low,  the  directional 
element  should  be  made  very  sensitive.  Consequently  small 
surges  on  the  system  may  frequently  cause  the  contacts  of  the 
directional  element  to  be  closed,  but  this  is  a  matter  of  no 
importance  because  these  contacts  are  also  in  series  with  the 
contacts  on  the  excess-current  element,  which  latter  contacts 
cannot  be  closed  unless  there  is  trouble  on  the  system 
with  a  consequent  excessive  current.  Now  if  the  excess-current 
element  of  this  relay  should  be  set  to  operate  at  a  very  low- 
current  value,  say  1  amp.,  and  the  load  current  is  3  amp.  these 
contacts  will  be  closed  continuously.  This  not  only  destroys  the 
time-limit  feature  of  the  relay,  but  places  the  service  at  the 
mercy  of  the  sensitive  directional  element,  which  is  likely  to 
operate  whenever  a  heavy  surge  occurs  on  the  system,  even  if 
this  surge  is  a  matter  of  no  importance  to  the  service. 

One  of  the  places  where  a  relay  with  a  low-current  winding 
has  been  used  is  on  city  distribution  systems  which  have  their 
own  stand-by  steam  plant,  but  which  purchase  part  of  their 


110  PROTECTIVE  RELAYS 

power  from  a  high-voltage  hydro-electric  system.  It  is 
obvious  that  the  small  steam  plant  cannot  hold  up  the  electric 
system  during  troubles  which  are  inherent  in  the  electric  system 
and  which  occur  altogether  too  frequently.  Nevertheless, 
even  if  the  receiving  system  should  have  sufficient  steam 
turbine  capacity  constantly  floating  on  the  line  to  pick  up  the 
electric  load  the  instant  it  is  lost,  such  a  change  cannot  be  made 
without  considerable  disturbance  and  there  is  always  a  possi- 
bility that  the  steam  plant  will  be  unable  to  pick  up  the  load 
as  quickly  as  is  necessary  in  order  to  save  the  service.  Now 
there  can  be  only  two  conditions  existing  at  the  steam  plant, 
either  there  is  sufficient  steam-generating  capacity  connected 
to  the  busbars  so  that  the  load  can  be  picked  up  instantly, 
and  if  this  is  the  case  there  is  sufficient  capacity  available  to 
operate  reverse-power  relays  having  a  normal-current  setting; 
or  else  there  is  insufficient  steam  capacity  to  pick  up  the  load, 
in  which  case  it  will  be  desirable  to  hold  on  to  the  electric  power 
through  whatever  manner  of  disturbance  may  occur. 

This  argument  is  usually  met  by  the  statement  that  the 
hydro-electric  system  frequently  "goes  under"  due  to  a  failure 
in  the  water  supply  or  due  to  short-circuit  on  the  transmission 
system  which  cuts  off  essential  plants.  Of  course,  under  such 
circumstances,  it  is  not  desirable  for  the  steam  plant  to  attempt 
to  carry  all  the  load  of  the  electric  system,  and  it  is  reasonable 
to  separate  the  two,  but  the  power-directional  relay  cannot 
be  depended  upon  to  make  this  separation  in  the  proper  manner. 
For  this  purpose  use  should  be  made  of  a  watt  relay  which  is 
essentially  a  contact-making  wattmeter  and  which  will  close 
its  contacts  when  the  power  exceeds  a  predetermined  amount 
in  a  predetermined  direction.  This  device  is  much  simpler  than 
the  power-directional  relay  and  can  be  depended  upon  to  oper- 
ate with  great  accuracy.  However,  it  is  not  intended  to  clear 
short-circuits,  and  in  the  installation  which  we  have  been  con- 
sidering, it  will  be  necessary  to  make  use  of  the  power-direc- 
tional relays  to  take  care  of  line  troubles  and  the  watt  relay 
to  prevent  the  steam  system  from  attempting  to  carry  the  hydro- 
electric system.  This  arrangement  will  give  complete  protec- 
tion, and  if  the  electric  system  is  in  itself  properly  sectionalized 
so  as  to  take  care  of  its  own  troubles,  there  will  be  very  little 


CHARACTERISTICS  OF  A.C.  DISTURBANCES  111 

necessity  for  keeping  the  stand-by  steam  plant  in  readiness 
to  instantly  carry  the  entire  load.  Viewed  from  this  stand- 
point the  question  becomes  of  considerable  importance  and  will 
justify  careful  consideration  on  the  part  of  prospective  relay 
users. 

Another  important  point  is  that  relays  with  a  low-current 
winding  not  only  have  a  high  impedance  but  they  will  not  carry 
a  heavy  load  without  severe  overheating. 

As  will  be  explained  later  under  the  pilot- wire  systems, 
low-current  relays  find  a  legitimate  application  when  the  currents 
must  be  transmitted  a  long  distance  between  the  current  trans- 
former and  relay.  In  this  case,  however,  use  is  made  of  current 
transformers  whose  normal  secondary  current  is  the  same  as 
the  current  rating  of  the  relay. 


CHAPTER  X 

INSTRUMENT  TRANSFORMERS  AND  GROUPINGS 

With  the  exception  of  the  smaller,  low-voltage  installations, 
where  the  relays  may  be  connected  directly  to  the  line  and  wound 
to  carry  the  line  current,  it  is  essential  to  connect  protective 
relays  to  the  secondary  circuit  of  instrument  transformers. 
It  then  becomes  necessary  to  have  an  accurate  knowledge 
of  the  performance  of  an  instrument  transformer  during  times 
of  electrical  trouble. 

Instrument  Transformers. — There  are  two  classes  of  instru- 
ment transformers  available:  the  " current"  or  "series"  trans- 
former and  the  "voltage,"  "potential"  or  "shunt"  transformer. 
The  current  transformers  are  used  to  carry  the  main-line 
current  in  their  primary  and  reproduce  in  their  secondary 
circuit  a  smaller  current  which  bears  a  definite  relation  in 
phase  and  magnitude  to  the  primary  current.  Another  very 
important  function  of  the  current  transformer  is  to  insulate 
the  secondary  circuit,  to  which  are  connected  the  relays,  from 
the  high  tension  of  the  primary. 

Current  transformers  are  required  for  relays  which  must 
function  on  a  predetermined  condition  of  the  current  in  a 
circuit.  The  primary  is  connected  directly  in  series  with  the 
line,  and  several  current  instruments  may  be  connected  to  the 
secondary.  Part  of  the  line  current  acts  as  the  magnetizing 
current  for  the  transformer  iron  and  for  a  fixed  number  of  instru- 
ments in  the  secondary;  a  rise  or  fall  in  the  line  current  requires 
a  corresponding  rise  or  fall  in  the  secondary  voltage  to  force 
the  secondary  current  through  the  connected  instruments. 
The  magnetic  flux  thus  follows  the  rise  and  fall  of  the  primary 
or  line  currents,  until  the  point  of  saturation  is  reached  when 
the  ratio  breaks  down. 

In  any  transformer,  the  primary  ampere-turns  may  be  con- 
sidered as  made  up  of  two  parts,  one  small  element  which 
supplies  the  magnetizing  and  core-loss  current,  and  another 

112 


INSTRUMENT  TRANSFORMERS  AN.D  GROUPINGS         113 

element  which  supplies  the  ''working  current."  The  " working 
current"  ampere-turns  are  always  exactly  equal  to  the  secondary 
ampere-turns. 

As  generally  used,  the  current  transformer  " steps  down" 
from  a  large  current  to  a  small  one,  so  that  the  primary  wind- 
ing consists  usually  of  few  turns  and  the  secondary  of  many 
turns. 

Inherent  Errors. — If  the  exciting  current  could  be  reduced 
to  zero,  the  secondary  ampere-turns  would  equal  the  primary 
ampere-turns  and  would  be  exactly  in  phase  opposition.  The 
transformer  would  then  produce  a  current  in  the  meters  which 
would,  under  all  conditions,  be  proportional  to  the  line  current 
and  in  exact  phase  opposition  to  it.  It  will  be  noted,  however, 
that  there  is  a  ratio  error  and  also  a  phase  displacement.  The 
losses  in  the  core  are  the  disturbing  elements  which  cause  both 
of  these  inherent  errors.  For  that  reason  the  cores  of  current 
transformers  should  be  made  of  generous  proportions  and  of  the 
best  magnetic  material,  so  that  the  density  of  the  magnetic  flux 
and  the  resulting  loss-elements  will  be  kept  as  low  as  possible. 

Ratio  Error. — If  the  exciting  current  decreased  exactly  in 
proportion  to  a  decrease  in  the  primary  current,  the  per  cent 
ratio  error  and  the  correction  factor  would  be  constant  for  all 
primary  currents.  However,  it  will  readily  be  seen  that  at 
a  flux  density  corresponding  to  one-half  normal  primary  current, 
the  exciting  current  is  more  than  one-half  of  its  value  at  full 
primary  current,  and  that  the  exciting  current  therefore  forms 
a  disturbing  element  of  relatively  large  proportions  as  the  pri- 
mary current  decreases. 

On  some  types  of  current  transformers,  especially  the  "hole" 
or  "through"  type,  the  ratio  breaks  down  badly  on  excessive 
overloads.  This  is  made  worse  by  the  fact  that  several  instru- 
ments may  be  connected  to  the  secondary.  For  this  reason 
many  systems  make  a  practice  of  .supplying  two  sets  of  trans- 
formers: one  for  the  metering  equipment  and  one  for  the  pro- 
tective equipment. 

As  an  example  of  how  the  ratio  breaks  down,  consider  the 
curve  in  Fig.  108.  This  shows  that  with  a  secondary  load  of 
40  v.  amp.,  should  the  short-circuit  current  be  20  times  normal, 
the  secondary  load  on  the  transformer  would  increase  from 


114 


PROTECTIVE  RELAYS 


40  v.  amp.  to  16,000  v.  amp.  and  this  in  many  cases  is  above 
the  saturating  point  of  the  transformer.  This  necessitates 
an  accurate  knowledge  of  the  ratio  of  the  transformer  with 
various  secondary  loads  and  various  primary  currents  in  order 
to  enable  the  relays  to  be  set  accurately. 


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Primary  Amperes 

FIG.   108. — Curve  showing  how  the  ratio  of  a  series  transformer  breaks  down  on 

heavy  overload. 


Magnetization  of  Core. — The  magnetic  history  of  the  iron 
also  affects  its  losses.  If  the  core  has  been  magnetized,  either 
by  passing  direct  current  through  the  coil,  by  opening  the  second- 
ary circuit  with  a  load  on  the  primary,  or  by  a  heavy  overload 
on  the  primary,  the  iron  loss  and  magnetizing  current  will  be 
abnormally  high  and  the  ratio  and  phase-angle  errors  will  be 
slightly  greater  than  normal.  Such  a  transformer  can  be  demag- 
netized and  restored  to  normal  condition  by  passing  about 
150  per  cent  of  normal  current  through  the  primary,  with  the 
secondary  connected  to  a  resistance  of  20  to  30  ohms  and  gradu- 


INSTRUMENT  TRANSFORMERS  AND  GROUPINGS       115 

ally  reducing  the  resistance  to  zero.  Great  care  should  be  taken 
not  to  come  in  contact  with  the  secondary  leads  during  this 
operation  as  dangerous  voltage  may  be  induced. 

Effect  of  Secondary  Load. — The  instruments  connected  in 
the  secondary  circuit  of  the  transformer  are  placed  in  series, 
so  that  the  secondary  current  will  pass  through  each.  As  instru- 
ments are  added,  higher  voltage  is  required  to  force  the  current 
through  them.  This  requires  higher  magnetic  density  in  the 
iron,  which  increases  both  the  iron  loss  and  the  magnetizing 
current,  hence  both  the  ratio  and  the  phase-angle  errors  are 
magnified.  For  the  sake  of  accuracy,  therefore,  there  is 
a  limit  to  the  number  of  instruments  that  should  be  placed  on 
a  single-current  transformer. 

The  ordinary  measuring  instruments  are  not  non-inductive. 
The  power  factor  of  the  load  of  instruments  varies  with  the 
different  combinations  used.  In  general,  and  within  the  limits 
of  the  usual  groups  of  meters,  it  may  be  said  that  for  the  same 
volt-ampere  load,  the  greater  the  inductive  element  in  the  load, 
the  less  will  be  the  phase  displacement  error  and  the  greater  the 
ratio  error.  While  the  variations  in  the  errors  are  not  enough 
to  affect  the  accuracy  to  a  great  extent,  the  power  factor  of  the 
load  must  be  recognized  in  preparing  performance  curves  of 
current  transformers. 

For  a  given  instrument  load  on  the  transformer,  the  secondary 
ampere-turns  bear  a  definite  relation  to  the  primary  ampere- 
turns,  for  each  value  of  the  primary-load  current.  Therefore, 
by  properly  proportioning  the  number  of  turns  in4he  windings, 
it  is  possible  to  raise  the  secondary  current  to  overcome  the 
ratio  error.  However,  owing  to  the  inherent  variation  of  the 
ratio  error,  this  compensation  will  not  be  exactly  correct  for 
other  values  of  the  primary  current. 

A  current  transformer  is  usually  compensated  to  give,  as 
closely  as  possible,  the  correct  ratio  at  65  per  cent  of  its  rated 
current.  As  meters  and  transformers  should  be  selected  with 
a  rating  50  per  cent  greater  than  the  normal  current  of  the 
circuit,  to  allow  for  peaks  and  overloads,  the  full-load  current 
of  the  circuit  represents  about  65  per  cent  of  the  current  rating 
of  the  transformer  and  meter.  Therefore,  the  greatest  accu- 


116  PROTECTIVE  RELAYS 

racy  of  meter  readings  is  attained  with  full-load  current  in  the 
circuit. 

Higher  frequencies  produce  lower  magnetizing  current  and  lower 
iron  loss,  and  therefore  result  in  lower  percentage  of  ratio  error 
and  smaller  phase  angle.  The  variations  are  small,  however, 
and  most  current  transformers  may  be  used  at  any  frequency 
from  25  to  133  cycles. 

As  the  operation  of  the  current  transformer  depends  on 
current  only,  variation  of  line  voltage  has  no  effect  on  accuracy. 
A  type  of  current  transformer  must  be  chosen,  however,  having 
insulation  suitable  for  the  voltage  of  the  line  on  which  it  is 
to  be  used. 

The  shape  of  the  primary-current  wave  affects,  to  a  certain 
extent,  the  maximum  induction  and  therefore  the  iron  losses, 
and  also  affects  the  shape  of  the  secondary-current  wave,  which 
may  introduce  slight  errors  in  some  meters.  These  effects 
are,  however,  negligible. 

Rise  of  temperature  increases  the  resistance  drop  in  the 
windings,  which  necessitates  an  increase  in  the  secondary  volt- 
age. This,  in  turn,  necessitates  an  increase  in  the  magnetic 
density  required  in  the  iron  and  thus  affects  the  accuracy. 
The  resistance  drop  is,  however,  only  a  small  part  of  the  induced 
voltage,  and  the  temperature  rise  of  transformers  should  be 
within  the  A.  I.  E.  E.  limit  of  55°C.  The  variations  of  accu- 
racy due  to  temperature  rise  are  very  slight. 

A  current  transformer,  to  be  accurate,  requires  at  least 
600  ampere-turns  (based  on  normal  primary  current).  In  the 
"  through-type"  there  is  only  one  primary  turn,  so  that  this 
type  cannot  be  made  for  normal  currents  of  less  that  600  amp. 
without  sacrificing  accuracy.  In  cases  where  accuracy  is 
required  over  only  a  limited  range,  as  for  relays  or  trip  coils, 
the  use  of  this  type  is  entirely  satisfactory  for  normal  current 
as  low  as  100  amp.  Where  it  is  possible  to  calibrate  the  instru- 
ment with  the  transformer,  it  is  entirely  satisfactory  to  use 
this  type  of  transformer. 

The  momentary  current  due  to  a  heavy  short-circuit  on  a 
large  system  is  extremely  great  and  the  mechanical  stresses  set 
up  between  the  primary  and  secondary  windings  of  a  current 
transformer  due  to  this  current  are  very  large.  In  the  "  through- 


INSTRUMENT  TRANSFORMERS  AND  GROUPINGS       117 

type"  of  transformer,  these  stresses  are  balanced  within  the 
transformer  itself.  This  is  a  good  type,  therefore,  to  apply 
where  there  is  a  liability  of  short-circuits. 

The  objection  has  been  put  forward  that  the  accuracy  of  the 
" through-type"  of  transformer  is  affected  by  the  position  of  the 
primary  conductor  in  the  transformer  opening.  This  would  be 
true  to  a  slight  extent  if  the  conductor  were  very  small  in  pro- 
portion to  the  transformer  opening.  In  practice  it  amounts  to 
a  laboratory  refinement  which  is  of  no  commercial  importance. 


To  Current 
Coil  of  Meter 


FIG.  109.  FIG.  110. 

FIG.   109. — Connections   of  three-wire  transformer  on  single-phase  three-wire 
circuit. 

FIG.   110. — Connections  of  two  transformers  on  single-phase  three- wire  circuit. 


Single-phase  Groupings. — For  single-phase  circuits  a  trans- 
former is  required  for  each  circuit  to  be  protected.  In  the  case 
of  three-wire  circuits,  either  two  ordinary  transformers  or  one 
three-wire  transformer  may  be  used,  connected  as  shown  in 
Fig.  109  and  Fig.  110.  The  three- wire  transformer  is  so 
connected  that  the  secondary  carries  current  proportional  to  the 
average  of  the  currents  in  the  outside  wires  of  the  circuit.  When 
two  single  transformers  are  used,  connected  like  Fig.  110,  the 
current  in  the  relay  circuit  is  the  sum  of  the  two  secondary 
currents.  To  use  standard  apparatus  throughout  it  would 
be  necessary  to  use  transformers  of  double  the  actual  ratio 
required  so  that  the  sum  of  the  secondary  currents  would  not 
exceed  the  5  amp.  for  which  the  relay "  coil  is  designed.  Such 
transformers  would  then  be  operating  at  one-half  their  normal 
primary  current,  and  their  accuracy  would  accordingly  suffer 
somewhat. 

Two -phase  Groupings. — Figure  111  shows  a  number  of 
possible  groupings  of  current  transformers  on  various  two- 
phase  circuits,  with  the  corresponding  vector  diagrams.  As  sum- 


118 


PROTECTIVE  RELAYS 


A     E    0 


A     £    D 


8 


(a)  (b) 

,    Three-wire.     One  transformer  in  each  phase.     V-connection. 
)   Three-wire.     Transformers  in  one-phase  and  common  wire.     V-Connection. 


A     8    C    D 


A     B    C    D 


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D  C 


B 


Primary       6  Secondonf 

(c).  Four-wire,  independent.    One  transformer  in  each  phase.    V-Connectfon. 
(d)   Four-wire,  inter-connected.    T-Conriection. 

K       B        C 


ftr/mory 


Fia.  111. — Two-phase  groupings  of  current  transformers  showing  connections 

and  vectors. 


INSTRUMENT  TRANSFORMERS  AND  GROUPINGS       119 


ing  that  the  transformers  have  standard  5-amp.  secondary 
windings,  the  numbers  on  the  vector  diagrams  show  the  currents 
in  the  corresponding  branches  of  the  circuits.  The  preferable 
arrangement  for  any  case  depends  on  the  type  of  instrument 
to  be  energized.  For  ammeters,  a  reading  in  each  phase  usually 
is  all  that  is  necessary,  while  protective  relays  should  be  so 
connected  that  trouble  on  any  line  will  be  detected. 

NECESSITY    FOR    THREE    TRANSFORMERS    ON    THREE-PHASE 

CIRCUITS 

That  the  use  of  two  transformers  and  two  relays  for  the  pro- 
tection of  a  three-phase  circuit  is  not  sufficient  is  apparent  from 
Fig.  1 12,  which  shows  the  relays  at  A  and  B  with  the  transformers 


\ 


FIG.   112. — Showing  why  two  series  transformers  are  not  sufficient  for  three. 

phase  protection. 

at  1  and  2.     If  a  ground  should  occur  on  line  2,  and  another 
ground  on  a  generator  winding  or  lead  as  at  a  or  b  or  c,  then 


I     Z     3 


FIG.  113.  FIQ.  114. 

FIG.   113. — Wrong  connection  of  three  transformers  and  two  relays. 
FIG.  114. — Another  wrong  connection. 

there  would  be  a  heavy  short-circuit  current  which  would 
flow  through  the  middle  line  without  passing  through  either 
relay. 


120 


PROTECTIVE  RELAYS 


Three  transformers  cannot  be  used  with  two  relays  with 
the  connections  as  shown  in  Fig.  113  as  it  will  be  noted  that 
in  the  event  of  a  short  between  lines  1  and  3,  there  will  be 
no  excess  current  in  the  relay.  In  Fig.  114,  a  short  due  to  ground 
on  either  lines  1  or  3  will  tend  to  force  the  excess  current  through 
the  middle  transformer,  thereby  piling  up  the  voltage  due  to 
the  high  impedance,  so  this  connection  also  cannot  be  used. 


3    2    I 


3     2 


FIG.  115.  FIG.  116. 

FIG.   115. — Proper  connection  for  three  relays  and  three  transformers. 
FIG.   116. — Delta  connection  to  two  relays. 

The  best  combination  is  three  transformers  with  three  relays 
as  in  Fig.  115  as  this  enables  each  relay  to  receive  the  full  current 
from  its  respective  transformer.  This  is  the  most  used  connec- 
tion. Two  relays  may  be  connected  to  three  transformers 
with  the  delta  connection  as  in  Fig.  116  or  with  theZ  connection 
as  in  Fig.  117. 


3      2.      I 


FIG.  117. — Z-connection  to  two  relays. 

Advantages  of  the  Z -Connection. — Assuming  that  the  normal 
current  in  the  transformer  secondary  is  5  amp.,  then  with  the 
delta  connection,  the  normal  current  in  the  relays  is  8.66  amp., 
thus  requiring  specially- wound  relays.  With  the  Z-connection, 


INSTRUMENT  TRANSFORMERS  AND  GROUPINGS       121 

however,  the  normal-relay  current  is  only  5  amp.  In  the  case  of 
a  short  between  two  lines,  the  currents  increase  as  shown  in  the 
table,  Fig.  118,  showing  that  both  delta  and  Z  give  equal  pro- 


FIG.  118. — TABLE  SHOWING  INCREASE  IN  CURRENT  WITH  Z  AND  DELTA 
CONNECTIONS.     OVERLOAD   IN  Two  LiNES1 


Trans. 

Relay 

Increase  in 

Per  cent, 

current 

current 

current 

ratio 

r, 

h 

Z 

A 

Z 

A 

Z 

A 

5 

5 

5 

8.66 

0 

0.0 

0 

0 

6 

6 

6 

10.39 

1 

1.73 

20 

20 

8 

8 

8 

13.86 

3 

5.20 

60 

60 

10 

10 

10 

17.32 

5 

8.66 

100 

100 

1  Alfred  J.  A.  Peterson  in  the  Electric  Journal. 

FIG.  119. — TABLE  SHOWING  INCREASES  IN  CURRENT  WITH  Z  AND  DELTA 
CONNECTIONS.     OVERLOAD  IN  ONE  LINE  ONLY* 


Trans. 

Relay 

Increase  in 

Per  cent, 

Per  cent, 

currents 

currents 

current 

increase 

ratio 

i\ 

13 

Z 

A 

Z 

A 

Z 

A 

Z 

A 

5 

5 

5.00 

8.66 

0.00 

0.00 

100.0 

173 

6 

5 

5.57 

9.54 

0.57 

0.88 

11 

10.15 

92.8 

159 

7 

5 

6.25 

10.43 

1.25 

1.77 

25 

20.4 

89.3 

149 

8 

5 

7.00 

11.35 

2.00 

2.69 

40 

31.0 

87.5 

142 

9 

5 

7.82 

12.30 

2.82 

3.64 

56 

42.0 

86.9 

137 

10 

5 

8.66 

13.23 

3.66 

4.57 

73 

53.0 

86.6 

132 

12 

5 

10.43 

15.13 

5.43 

6.47 

109 

75.0 

86.9 

126 

15 

5 

13.23 

18.04 

8.23 

10.38 

165 

120.0 

88.2 

120 

20 

5 

18.04 

22.94 

13.04 

14.28 

361 

165.0 

90.2 

115 

1  Alfred  J.  A.  Peterson  in  the  Electric  Journal. 

tection.  But  in  the  case  of  a  three-phase  four- wire  system  or 
a  grounded  neutral  system,  a  short-circuit  to  neutral  or  ground 
will  cause  the  currents  to  increase  as  in  table,  Fig.  119,  thus 
showing  that  the  Z-connection  gives  much  better  protection. 


122  PROTECTIVE  RELAYS 

When  making  the  Z-connection,  the  following  rule  should 
be  employed:  Connect  two  positive  transformer  terminals 
to  the  first-relay  terminal.  The  negative  terminals  of  the 
first  and  third  transformer  go  to  the  second-relay  terminal  and 
the  remaining  positive  and  negative  transformer  terminals 
go  to  the  common  between  the  two  relays.  Thus  referring 


FIG.  120. — VARIOUS  Z  COMBINATIONS 
LINE 
3-J 


•4-1 


LOAD 


RELAY  A 

RELAY  B 

RELAY  C 

1-3 

4-5 

2-6 

1-3 

3-6 

2-4 

1-2 

4r-Q 

3-5 

1-2 

5-6 

3-4 

2-3 

4-6 

1-5 

2-3 

4-5 

1-6 

to  Fig.  120  it  will  be  seen  that  there  are  six  combinations  which 
produce  the  same  results.  Figure  121  gives  the  various  groupings 
and  vector  diagrams. 

Opening  of  Secondary. — The  secondary  circuit  of  a  current 
transformer  should  never  be  opened  while  the  primary  is  carry- 
ing current.  If  it  is  necessary  to  disconnect  instruments  the 
secondary  should  first  be  short-circuited.  If  the  secondary 
circuit  is  opened,  a  difference  of  potential  is  developed  between 
terminals  which  is  dangerous  to  anyone  coming  in  contact 
with  the  meters  or  leads.  The  cause  of  this  high  voltage  is 
that  with  open  secondary  circuit  all  of  the  primary  ampere- 
turns  are  effective  in  producing  flux  in  the  core,  whereas  normally 
but  a  very  small  portion  of  the  total  perform  this  function.  The 
danger  is  magnified  by  the  fact  that  the  wave  form  of  this 
secondary  voltage  is  peaked,  producing  a  high-maximum  value. 
A  high  flux  produced  in  this  way  may  also  permanently  change 
the  magnetic  condition  of  the  core  so  that  the  accuracy  of  the 
transformer  will  be  impaired.  V 

Voltage  Transformers. — Voltage  transformers  (ajso  called 
potential  or  shunt  transformers)  are  used  to  insulate  the  relay 
circuit  from  the  high-tension  line  circuit,  and  to  reproduce 
a  voltage  on  the  relays  which  is  in  direct  proportion  to  the  line 
voltage. 


INSTRUMENT  TRANSFORMERS  AND  GROUPINGS       123 


ABC 


ABC 


Z 

(a)    Delta  Connection 


ABC 


(c)  Vector  parrallel 


8    C 


kc\r 
" — 


Primary  u  Secondary 
(d)   Reversed  V 


ABC 


tz 


(e)    Z-Connection 

FIQ.  121. — Three-phase  groupings  of  current  transformers  showing  connections 

and  vectors. 


124  PROTECTIVE  RELAYS 

This  transformer  is  in  principle  of  operation  an  ordinary 
constant-potential  transformer  specially  designed  for  close 
regulation,  so  that  the  secondary  voltage  under  any  conditions 
will  be  as  nearly  as  possible  a  fixed  percentage  of  the  primary 
voltage. 

The  only  thing  which  can  affect  the  accuracy  of  a  voltage 
transformer,  without  entirely  destroying  it,  is  a  change  in  the 
iron  which  would  change  the  exciting  current.  Inasmuch  as  the 
effect  of  the  exciting  current  is  small,  and  modern  transformer 
iron  is  non-aging,  it  is  safe  to  assume  that  the  original  calibra- 
tion of  a  first-class  modern  transformer  is  permanent. 

Voltage  transformers  are  compensated  for  their  iron  losses 
at  their  rated  voltage.  When  used  on  some  other  voltage, 
either  higher  or  lower,  an  error  is  introduced.  In  general, 
this  error  will  not  be  more  than  0.15  per  cent  when  the  applied 
voltage  is  from  50  per  cent  to  110  per  cent  of  rated  voltage. 
A  voltage  transformer  should  never  be  used  on  a  circuit  whose 
voltage  is  more  than  10  per  cent  above  the  rated  voltage  of 
the  transformer. 

Ordinary  frequency  variation  and  wave  shape  also  affect 
the  iron  losses,  but  their  effects  on  the  accuracy  of  the  trans- 
former cannot  be  detected. 

As  the  operation  of  the  voltage  transformer  depends  only 
on  the  voltage  applied  at  its  terminals,  variations  in  the  line 
current  have  no  effect  whatever  on  its  accuracy. 

As  voltage  transformers  are  designed  for  close  regulation, 
they  should  have  a  temperature  rise  well  within  the  A.  I.  E.  E. 
limit  of  55°C. 

Polyphase  Groupings. — In  general,  two  voltage  transformers 
are  sufficient  for  any  two-phase  or  three-phase  circuit.  Figures 
122  and  123  show  various  groupings  of  transformers  on  two- 
phase  and  three-phase  circuits  respectively.  The  numbers 
shown  on  the  vector  diagrams  of  secondary  connections  show 
the  voltage  between  the  points  indicated,  in  percentage  of  the 
voltage  between  lines  (corrected  for  ratio  of  the  transformers). 
In  case  a  different  secondary  voltage  between  these  points  is 
desired,  transformers  of  suitable  ratio  should  be  selected.  The 
highest  accuracy  is  attained  with  standard  transformers  when 
the  secondary  voltage  of  the  transformers  is  100  v.,  but 


INSTRUMENT  TRANSFORMERS  AND  GROUPINGS       125 


in  some  cases  58  v.  is  required  to  produce  proper  registration 
of  the  relays. 

It  should  be  noted  that  the  F-primary  to  F-secondary  connec- 
tion (6),  Fig.  123,  produces  the  same  results  as  the  "delta-delta" 
connection  (a)  and  saves  one  transformer.  The  "delta-delta " 
connection  is  therefore  seldom  used  commercially.  The  F-F 


E     C 


C      B      D 


E  C  e  c  °  *  ^a 

Primary  Secondary  ™*»  Secondary 

FIG.  122. — Two-phase  groupings  of  voltage  transformers  showing  connections 

and  vectors. 

connection  under  some  conditions  produces  slightly  higher 
error,  but  the  difference  is  not  considered  of  sufficient  impor- 
tance to  warrant  the  expense  of  the  extra  transformer. 

To  check  the  correctness  of  the  connections  of  voltage  trans- 
formers on  three-phase  lines,  three  voltage  readings  are  neces- 
sary. These  three  voltage  readings  should  bear  the  relations 
shown  in  the  diagram. 

The  secondary  terminals  of  a  voltage  transformer  should  never 
be  short-circuited.  If  they  should  become  short-circuited,  a 
heavy  current  will  flow  which,  if  continued,  will  burn  out  the 
windings. 

It  is  practically  impossible  to  protect  thoroughly  a  voltage 
transformer  with  fuses,  for  the  reason  that  any  fuse  wire  small 
enough  and  long  enough  to  open  the  transformer  circuit  with 
certainty  during  an  overload  would  be  mechanically  too  frail 


126 


PROTECTIVE  RELAYS 


N  A 


N  A  B  C 


a 

'  Primary  Scconoonf 

(c)    V-ReversedV 


N  A  BC 


Primary 
(e)    Open  Y-Reversed  open  Y 


fa 

•*ary 

C 
J 

0    100    C 

Secondary 
a)    Delta-Delta 

A 

P-  .-• 

£ 

a  too" 

Secondary 
(b)    V-V 

V, 

f     , 

9  < 

:                                ( 

n 

V  , 

4  t 

3  ( 

z??i 

C 

^£_ 

N 


n 


Primary 
(d)   OpenY-OpenY 


N  A  BC 


100       c 

Secondary 


Primary  Seontary 

(f)     Y-Y 


FIG.  123.  —  Three-phase  groupings  of  voltage  transformers  showing  connections 

and  vectors. 


INSTRUMENT  TRANSFORMERS  AND  GROUPINGS       127 

to  be  handled.  Some  companies  have  adopted  the  practice 
of  connecting  the  voltage,  transformers  directly  to  the  lines 
without  fuses.  This  is  dangerous,  because  a  short-circuit 
within  the  transformer  might  cause  a  high-voltage  lead  to 
burn  off  and  fall  in  such  a  way  as  to  short-circuit  the  system. 
To  prevent  this  the  larger  American  electrical  manufacturers 
recommend  the  use  of  resistors  and  fuses  in  the  high-voltage 
leads  of  voltage  transformers.  The  resistors  limit  the  short- 
circuit  current  to  20  to  40  amp.,  while  the  fuses  are  designed 
to  open  such  a  current. 

In  normal  operation,  the  resistors  carry  only  the  very  small 
primary  current  of  the  voltage  transformer,  and  the  drop  in 
voltage  which  they  cause  is  inappreciable. 

Load  on  Transformers. — If  several  instruments  are  connected 
to  the  same  transformer,  the  combined  load  may  be  found  as 
follows:  Let  TFi,  Wz,  W3,  etc.  be  the  true  watts  required  by  the 
several  instruments. 

And  Mi,  M2,  M&,  etc.  be  the  magnetizing  reactive  volt-amperes 
required  by  the  several  instruments. 

Then  the  volt-ampere  load  on  the  secondary  of  the  trans- 
former will  be 

L  =  V7w7+  W  2  +  W  3  + )2  +  (Ml  +  M2  +  Ms  + )2 

and  the  power  factor  of  this  secondary  will  be 

W !  +  W,  +  W  a  +  .... 
PF  ~L~ 

These  relations  are  true  in  single-phase  or  two-phase  systems 
where  the  current  from  each  transformer  flows  through  its 
own  load.  As  an  approximation  which  is  fairly  close,  the 
volt-amperes  of  the  secondary  load  may  be  taken  as  the  sum 
of  the  volt-amperes  of  the  several  instruments.  And  the  power 
factor  of  the  secondary  load  may  be  taken  as  the  sum  of  the 
watts  divided  by  the  sum  of  the  volt-amperes. 

Three-phase  circuits  having  a  set  of  transformers  for  each 
phase  are  approximately  equivalent  to  three  single-phase 
circuits,  and  the  transformer  error,  calculated  as  for  a  single- 
phase  system,  will  be  the  average  error.  But  when  only  two 
transformers  are  used  on  a  three-phase  system,  the  calculation 


128  PROTECTIVE  RELAYS 

of  the  loads  on  the  individual  transformers  becomes  more 
complicated  and  is  not  included  here.  When  accuracy  is 
required  such  that  exact  correction  for  phase-angle  and  ratio 
is  necessary,  two  transformers  should  not  be  used  on  three-phase 
systems. 

RELAYS  REQUIRING  BOTH  CURRENT  AND  POTENTIAL 
TRANSFORMERS 

On  single-phase  and  two-phase  circuits,  current  and  poten- 
tial transformers  may  be  connected  to  the  relays  according 
to  the  foregoing  directions.  But  when  relays  requiring  both 
current  and  potential  are  used  on  three-phase  circuits,  then 
special  provision  must  be  made  to  maintain  the  correct  phase 
relations,  or  rather  the  phase  relations  demanded  to  give  adequate 
protection. 

For  ordinary  watt  protection,  the  relays  must  be  connected 
so  that  they  always  give  positive  deflection  with  the  load  in 
a  given  direction.  This  excludes  the  connection  usually  used 
with  two  single-phase  wattmeters  on  three-phase.  As  is  well 
known,  the  voltage  and  current  on  one  instrument  falls  90  deg. 
out  of  phase  at  50  per  cent  line-power  factor  and  the  deflection 
actually  reverses  below  this  value.  The  correct  connections 
are  shown  in  Fig.  124. 

With  directional  relays  which  are  used  to  sectionalize  and 
isolate  a  short-circuited  line,  it  was  once  customary  to  use 
the  star  connection  for  current  transformers  and  the  star- 
delta  connection  for  voltage  transformers  as  shown  in  Fig.  125. 

This  caused  the  relay  current  and  voltage  to  be  in  phase 
at  100  per  cent  line-power  factor.  This  scheme  is  still  used, 
particularly  on  underground  cable  systems,  where  the  resistance 
is  high  compared  to  the  inductance.  But  on  long,  overhead 
lines  and  in  every  case  where  feeder  reactors  are  used,  a  short- 
circuit  may  be  of  such  low  power  factor  that  there  will  not 
be  enough  energy  to  cause  the  directional  relays  to  function. 

One  method  of  curing  this  trouble  is  very  simple:  The 
relays  should  be  connected  with  the  voltage  coils  across  the 
same  conductors  which  are  causing  the  short-circuit.  In 
other  words,  the  voltage  coils  should  be  connected  in  delta  in 
accordance  with  Fig.  126.  Because  the  current  will  lag  behind 


INSTRUMENT  TRANSFORMERS  AND  GROUPINGS       129 


the  voltage  when  a  short-circuit  occurs,  the  connection  should 
be  so  made  that  at  unity  power  factor  the  current  in  the  current 
coils  of  the  relays  will  lead  the  voltage  by  30  deg.  This  con- 


3  PHASE  LINE 


(A)     GROUND 


~U 

TO  D.C.  CONTROL 
CIRCUIT 


3  PHASE  LINE 


CURRENT 
TRANSFORMERS* 


TO  DECONTROL 
CIRCUIT 


3  PHASE  LINE 


CURRENT 
TRANSFORMERS* 


GROUND        '    '    '  (C) 

FIG.   124. — Connections  for  watt  relays  as  recommended  by  Westinghouse  Co. 


TO  DECONTROL 
CIRCUIT 


nection  not  only  overcomes  the  trouble  from  distortion,  but 
it  allows  the  relays  at  all  times  to  operate  under  a  higher  power 
factor. 

There  are  two  very  simple  methods  of  determining  the  cor- 
rect connection.     One  is  to  connect  the  current  coils  of  a  single- 


130 


PROTECTIVE  RELAYS 


phase  power-factor  meter  in  series  with  the  relay-current  circuit, 
and  then  with  100  per  cent  power  factor  on  the  line,  select  the 


9000 


gft     &•  9000  volts 
y     §9000  volts 


BUQ  bars 


% 

1    1 

'   C 

> 

i 

\ 

i 

p 

{ 
1 

I 

f 

1 

I 

terminals 

1 

1 

li 

1 

Load 


FIG.  125. — Star-delta  relay  connection,  in  which  the  current  is  in  phase  with 
the  voltage  at  100  per  cent  line  power  factor. 


/Pe/cry 


Pe/ay 


FIG.  126. — Connections  of  relays  to  cause  the  current  to  lead  the  voltage  on 

non-inductive  loads. 

pair  of  voltage  leads  which  give  about  86  per  cent  power-factor 
lead  on  the  meter. 


INSTRUMENT  TRANSFORMERS  AND  GROUPINGS       131 

The  second  method  is  to  use  a  single-phase,  indicating  watt- 
meter, and  with  a  lagging  power  factor  on  the  line  between 
50  and  100  per  cent  select  the  pair  of  voltage  leads  which  give 
the  highest  reading. 

Just  after  making  the  above  test  is  the  proper  time  to  see 
that  the  contacts  are  held  open.  If  they  close,  the  voltage  leads 
must  be  reversed. 

The  above  discussion  is  not  based  solely  upon  the  mathe- 
matical study  of  the  problem,  but  is  the  result  of  actual  tests 
made  on  a  number  of  transmission  lines  where  the  reverse- 
energy  relays  connected  according  to  the  old  method  have 
not  given  satisfactory  service.  Experiments  have  shown  that 
this  method  of  connection  should  also  be  used  on  systems  having 
a  grounded  neutral.  This  connection  (with  the  current  30 
deg.  ahead  of  the  voltage)  must  be  used  with  care  on  an 
ungrounded  neutral  system  having  a  heavy  charging  current  to 
ground.  Difficulty  may  also  be  encountered  on  some  systems 
where  the  load  current  is  leading.  But  in  both  these  cases  the 
short-circuit  currents  will  be  much  greater  than  any  possible 
leading  current  and  no  difficulty  due  to  incorrect  operation  of 
the  reverse-power  relays  will  be  experienced  if  the  excess- 
current  elements  are  adjusted  to  operate  only  on  short-circuits. 

Determining  Phase  Rotation. — In  order  to  function  correctly, 
the  directional  relays  must  have  the  current  lead  30  deg.  and  not 
lag  30  deg.,  and  in  order  to  obtain  this  condition  it  is  necessary 
to  determine  the  phase  rotation.  This  cannot  be  determined 
from  an  ordinary  inspection  of  the  three  wires,  but  is  easily 
determined  by  small  patented  devices  now  on  the  market  or 
by  a  simple  apparatus  consisting  of  two  ordinary  incandescent 
lamps  and  a  suitable  reactor  connected  in  Y.  The  reactor 
should  have  about  the  same  reactance  as  the  lamps  have 
resistance. 

Calling  the  three-phase  voltage  wires  A,  B,  and  C,  connect 
one  lamp  to  A  and  the  other  to  C,  and  connect  the  reactor  to 
B.  One  lamp  will  now  burn  bright  and  one  dim  and  the  rule 
is  that  the  bright  light  always  leads  the  inductance.  For 
instance,  if  the  lamp  connected  to  A  should  be  bright,  the  phase 
rotation  is  A  B  (7,  while  if  the  lamp  connected  to  C  should 
be  bright,  then  the  phase  rotation  would  be  C  B  A. 


CHAPTER    XI 

PROTECTION  OF  MOTORS,  TRANSFORMERS 
GENERATORS,   AND   LINES 

In  the  early  days  of  electric  service,  protection  of  motors 
and  transformers  was  accomplished  by  ordinary  fuses,  which 
disconnected  the  apparatus  automatically  when  the  current 
became  excessive.  Even  today,  fuses  cannot  be  excelled  for 
reliability  on  heavy  overloads.  It  was  soon  realized  that  there 
were  conditions  of  overload  which  the  fuse  did  not  take  care 
of  adequately,  and  besides,  the  fuse  was  not  at  all  accurate  or 
selective  in  its  action  and  was  quite  expensive.  For  this 
reason  the  circuit  breaker  was  developed,  but,  while  it  was 
a  great  improvement,  yet  it  possessed  small  " reasoning"  or 
"thinking"  power;  that  is,  it  was  very  little  better  than  the 
fuse  in  its  selective  action.  This  led  to  the  use  of  the  protec- 
tive relay,  a  small  instrument  actuated  by  the  currents  in  the 
machines  or  wires  and  controlling  the  action  of  the  breaker. 
The  relay  is  so  flexible  in  its  various  connections  and  so  accurate 
and  selective  in  its  action  that  it  is  often  called  the  " brains" 
of  an  electric  system. 

While  the  principal  use  of  the  protective  relay  is  on  large 
generating  systems  and  long-distance  transmission  lines,  yet 
it  is  often  applied  to  motors  and  transformers  and  gives  protec- 
tion and  uninterrupted  service  that  can  be  obtained  in  no  other 
way. 

Protection  of  Motors. — In  its  simplest  application,  the  relay 
is  arranged  to  carry  a  current  proportional  to  the  load  current, 
and,  upon  the  occurence  of  excess  current,  close  a  circuit  which 
trips  or  opens  a  circuit  breaker.  Figure  127  shows  a  relay 
A  connected  to  the  secondary  of  a  series  transformer  B.  The 
primary  is  connected  to  carry  the  load  current  of  motor  C 
which  is  protected  by  breaker  D.  Normally  the  relay  contacts 
are  open,  but  should  motor  C  be  overloaded  or  develop  a  short- 

132 


MOTORS,  TRANSFORMERS,  GENERATORS,  AND  LINES    133 

circuit,  the  excess  current  operates  the  relay,  closing  the  contacts, 
which  in  turn  complete  the  circuit  to  the  trip  coil  of  the  breaker. 
The  time  between  the  instant  of  overload  and  the  instant  of 
breaker  opening  may  be  set  easily  by  the  time  lever  on  the  relay, 
for  instance,  referring  to  the  curves,  Fig.  171,  if  the  transformer 
delivers  5  amp.  at  full  load,  and  No.  10  setting  5  amp.  tap  is 
used,  then  on  150  per  cent  load,  it  may  take  5  or  6  sec.  to  trip, 
this  being  an  indefinite  part  of  the  curve. 


FIG.   127. — Simple  overload  protection. 

On  200  per  cent  of  load  (100  per  cent  overload),  it  will  take 
4  sec.  to  trip.  If  a  shorter  time  was  desired,  setting  No.  3  might 
be  chosen  and  the  same  load  would  be  tripped  out  in  1  sec. 
Should  the  load  be  over  700  per  cent,  No.  10  setting  would  trip 
out  in  2  sec.,  No.  5  setting  in  1  sec.  and  so  on. 

Suppose  it  was  desired  to  have  the  motor  trip  on  120  per 
cent  load  (20  per  cent  overload)  then  the  4-amp.  load  tap  would 
be  chosen.  At  120  per  cent  load  the  current  would  be  6  amp. 
(5  amp.  X  120  per  cent),  but  6  amp.  is  150  per  cent  load  on 
the  4-amp  tap,  so  the  relay  would  trip  out  in  6  sec.  on  No. 
10  setting,  and  quicker  on  heavier  overload  and  lower  settings. 

Relays  and  Transformers  Required. — If  a  two-phase  motor 
is  to  be  protected,  two  relays  and  two  transformers  are  necessary, 
unless  an  interconnected  system  is  used,  when  four  transformers 
must  be  employed.  One  single,  four-pole  circuit  breaker 


134 


PROTECTIVE  RELAYS 


FIG.  128. — Simple  protection  on  two-phase  circuit. 


FIG.   129. — Simple  overload  protection  on  a  three-phase  circuit. 


MOTORS,  TRANSFORMERS,  GENERATORS,  AND  LINES    135 

would  in  this  case  be  used  and  may  be  operated  with  one  or 
two  trip  coils. 

Figure  128  shows  a  two-phase  protected  motor  circuit.     If 
two  trip  coils  were  used,  they  might  be  connected  in  parallel, 


FIG.  130. — Standard  connections  of  Westinghouse  induction  relays  for  pro- 
tecting circuits  from  overloads.  In  all  cases  the  trip  circuit  must  be  opened 
by  an  auxiliary  pallet  switch  on  the  circuit  breaker. 

or  connected  separately  to  their  respective  relays.     In  any  case, 
both  phases  should  be  opened. 

Three-phase  motors  may  be  protected  with  two  relays, 
but  require  three  transformers  for  adequate  protection,  as  there 


136 


PROTECTIVE  RELAYS 


is  danger  of  an  overload  on  any  wire  should  the  motor- 
insulation  to  ground  fail.  The  reasons  for  this  are  discussed 
under  "  Instrument  Transformers  and  Groupings."  One  trip 
coil  is  generally  used,  tripped  by  either  relay,  but  the  breaker 
must  open  all  three  lines.  Figure  129  shows  the  connections. 


O.  C-  Operating  Bus 


OilCircu/t 
Breaker 


Tr/pCoil 


Auxiliary  Switch 
Relay 


Source 

FIG.  131. — Connections  of  G.  E.  relay  for  single-phase  circuit  protection. 

As  in  the  case  of  the  single  phase,  the  time  delay  in  tripping  may 
be  easily  set  by  the  time  lever  and  load  taps  on  the  relay.  Figure 
130  shows  typical  diagrams  of  connections  as  supplied  by  the 
manufacturer.  Additional  diagrams  are  shown  in  Figs.  131  to  135. 
Protection  of  Synchronous  Motors. — The  problem  of  pro- 
tecting a  synchronous  motor  or  condenser  is  peculiar  in  that 
it  is  desirable  to  have  the  motor  stay  on  the  line  just  as  long  as 
possible  in  the  event  of  external  trouble,  and  yet  it  should  be 
quickly  disconnected  in  case  of  internal  trouble.  The  motor 


MOTORS,  TRANSFORMERS,  GENERA  TORS,  AND  LINES    137 

must  be  capable  of  standing  heavy  overloads  for  a  short  time 
until  overheating  occurs  and  yet  it  must  be  protected  from  very 
severe  overloads.  Many  schemes  have  been  tried  with  varying 
success,  but  about  the  best  protection  seems  to  be  obtained  by 
using  an  overload  relay  with  special  time  characteristics  and 


B2  A2  Bl  Al 


&.  C.  Operating  0ua 


6     o 

Oi/ Ci r.  Breaker 


rt/9e 


Auxiliary  Switch 
Trip  Coll 


Current 
4  I  Transformer 


Source 


FIG.   132. — Connections  of  G.  E.  relay  for  two-phase  protection. 


temperature-load  relays,  which  are  more  fully  discussed  under 
" Temperature  Relays"  in  the  chapter  on  " Miscellaneous 
Relays." 

The  overload  relay  employed  is  similar  to  those  previously 
described,  but  it  has  in  addition  an  instantaneous  trip  attach- 
ment which  operates  at  about  500  per  cent  of  normal  load. 


138 


PROTECTIVE  RELAYS 


This  attachment  is  also  provided  with  a  calibrated  scale,  so 
that  it  may  be  set  within  wide  limits.  The  relay  proper  is  set 
for  a  very  long  time  delay  on  moderate  overloads  of  200  to  400 
per  cent  so  that  a  maximum  time  delay  is  obtained  in  mild  cases 
of  distress.  But  should  the  load  exceed  500  per  cent  then  there 
is  no  longer  time  to  delay,  so  the  instantaneous  trip  functions 


C<  Operating  Bu 


FZtse 

Aux/ftary  Switch 


FIG.  133. — Connections  of  G.  E.  relays  for  protection  of  a  three-phase  circuit 
with  ungrounded  neutral. 

and  cuts  out  the  motor  instantly.  This  arrangement  allows 
the  main-line  relays  to  isolate  defective  feeders  before  the  syn- 
chronous motor  is  thrown  off,  but  it  provides  instantaneous 
protection  should  a  short-circuit  develop  in  the  motor  or  its 
connecting  leads. 

Additional  protection  should  be  provided  for  in  the  form  of 
temperature  load  relays.  This  scheme  employs  exploring  tem- 
perature coils  built  right  in  the  stator  slots.  It  allows  the  machine 


MOTORS,  TRANSFORMERS,GENERATORS,  ANDLINES    139 


to  carry  a  heavy  overload  until  overheating  occurs.  Even  then 
though  the  machine  be  hot,  if  the  load  has  decreased,  the  relay 
will  not  function;  it  requires  both  high  temperature  and  high  load 
to  operate  it  and  then  it  usually  warns  the  operator  by  means 
of  an  alarm  or  it  operates  automatic  equipment  to  relieve 
the  load.  This  relay  is  more  fully  described  under  "Miscella- 
neous Relays." 


3    2     I 


D.C.  Operating  Bus 


Oi/Cir.6rM 


Auxiliary  Switch 
Trip  Coil  fie  lays 


FIG.  134. — Connections  of  G.  E.  relays  for  protection  of  a  three-phase  circuit 
with  a  grounded  neutral. 

Protection  of  Rotary  Converters. — Rotary  converters  should 
be  protected  on  both  A.C.  and  D.C.  side,  unless  the  rotary 
is  operating  an  isolated  line  without  storage-battery  stand-bys 
or  any  other  chance  of  the  current  reversing  and  motoring 
the  rotary.  On  the  A.C.  end,  there  should  be  provided  the  usual 
current-overload  relays  to  protect  against  severe  overloads. 
A  low-voltage  relay  should  be  provided  to  disconnect  the  rotary 
in  case  of  line- volt  age  failure. 


140 


PROTECTIVE  RELAYS 


The  protection  against  reversal  of  current  in  the  D.C.  end  has 
been  fully  described  under  "  Applications  of  D.C.  Power  Direc- 
tional Relays."  Every  rotary  should  be  provided  with  an 
overspeed  device  of  the  centrifugal  type  which  provides 
protection  should  the  other  devices  not  protect  it  from  over- 
speeding. 


D.C.  Operating  Bus 


Relays 


Current 
Transformer 


Source 


FIG.   135. — Connections  of  G.  E.  relays  for  protection  of  a  three-phase  four-wire 
circuit  with  or  without  a  grounded  neutral. 


A  complete  diagram  of  the  A.C.  and  D.C.  protective  relays 
is  shown  in  Fig.  136. 

Transformer  Protection. — If  the  relays  are  connected  on 
the  line  side  (generating  side)  of  transformers  which  operate 
a  load  of  motors,  etc.,  they  not  only  protect  the  load,  but  the 
transformers  as  well,  opening  the  circuit  in  case  of  trouble 
in  the  transformers. 

But  the  object  is  not  only  to  protect  but  also  to  preserve 


MOTORS,  TRANSFORMERS,  GENERATORS,  AND  LINES    141 

the  continuity  of  service,  if  possible.  So,  in  important  places, 
instead  of  using  one  set  of  large  transformers,  two  or  three 
sets  of  smaller  transformers  are  used.  For  instance,  instead 
of  using  a  1,200-kva.  transformer,  three  400-kva.  transformers 
might  be  used,  connected  in  parallel  or  "banked."  This  bank 
has  the  same  capacity  as  the  big  one:  but  if  one  set  goes  bad, 
the  other  two  in  the  bank  will  assume  the  load,  while  if  the  larger 


FIG.   136. — Illustrating  the  use  of  relays  in  protecting  a  rotary  converter. 

one  is  damaged,  the  service  is  interrupted  until  repairs  or  replace- 
ment can  be  made. 

It  is  the  function  of  the  relay  instantly  to  locate  trouble  in 
a  transformer  bank  and  cut  out  the  defective  unit  without 
interruption  to  the  load.  The  elementary  principle  of  differ- 
ential transformer  protection  is  shown  in  Fig.  137,  where  A 
is  a  large  power  transformer  having  a  10  to  1  voltage  ratio, 
full  load  current  100  amp.  in  the  primary  and  1,000  amp.  in 


142 


PROTECTIVE  RELAYS 


the  secondary.  Current  transformers  are  chosen  for  the  primary 
and  secondary,  which  give  the  same  secondary  current  (generally 
5  amp.).  These  are  shown  at  B  and  C.  The  secondaries 


FIG.  137. — Differential  transformer  protection. 


FIG.  138. — Showing  currents  in  relay  upon  short-circuit  in  transformer. 

are  connected  in  series  so  that  at  normal  load  and  correct  direc- 
tion of  power  flow,  a  current  will  circulate  In  the  secondaries 
and  none  in  the  relay,  as  there  is  as  much  tendency  for  flow 
in  one  direction  as  in  the  other. 


MOTORS,  TRANSFORMERS,  GENERA  TORS,  AND  LINES    143 

As  long  as  current  flows  away  from  the  transformer  in  the 
same  ratio  as  it  flows  into  it  (i.e.,  I  :10),  there  can  be  no  current 
in  the  relay;  but  consider  Fig.  138.  A  short-circuit  has  occurred 
in  the  winding  at  X  and  a  heavy  current,  say,  500  amp.,  is 
flowing  into  the  primary.  The  secondary  current  may  not 
drop  off  much,  but  the  transformer  would  soon  be  burned  out. 


FIG.  139. — Two  transformers  in  parallel ;  short-circuit  in  one. 

Consider  the  current  in  the  secondary  of  the  series  trans- 
formers. Five  hundred  amperes  on  a  100  to  5  transformer 
give  25  amp.  on  the  secondary.  The  other  transformer  (1,000 
to  5)  will  allow  only  5  amp.  to  flow  through  it,  so  the  extra 
20  amp.  must  go  somewhere.  Consequently,  they  flow  through 
the  relay,  close  the  contacts,  and  trip  both  the  breakers,  effectually 
isolating  the  transformer. 

Protection  in  Banks. — If,  however,  there  is  another  trans- 
former operating  in  parallel  with  this  and  a  short-circuit  occurs, 


144  PROTECTIVE  RELAYS 

the  good  transformer  may  feed  the  bad  transformer  from  the 
secondary  side  and  cause  an  actual  reversal  of  power  in  the 
defective  unit,  that  is,  power  feeds  into  the  transformers  from 
both  sides  as  will  be  seen  readily  by  a  consideration  of  the 
arrows  denoting  the  direction  of  instantaneous  current  flow 
in  Fig.  139. 

This  shows  that  the  good  transformer,  as  well  as  the  defective 
one,  becomes  heavily  overloaded  and  plainly  shows  that  plain 
overload  protection  would  cut  out  both  transformers  in  equal 
time.  The  differential  connections,  however,  cut  out  only 
the  defective  unit  and  throw  the  full  load  on  the  other  remain- 
ing units.  Then  if  the  load  is  too  great  to  be  safely  carried, 
the  overload  relays  must  necessarily  cut  out  the  good  unit 
to  prevent  it  from  damage,  which  results  in  unavoidable  inter- 
ruption. This  would  seldom  occur;  for  example,  in  our  previous 
example  of  three  transformers  carrying  a  load  of  1,200  kva., 
if  one  goes  bad,  the  others  must  each  carry  a  load  of  600  kva., 
which  is  only  50  per  cent  overload,  and  should  be  carried  without 
difficulty.  Should  two  transformers  go  bad,  however,  it  is 
obvious  that  one  transformer  of  400  kva.  could  not  carry  a 
load  of  1,200  kva.  and  the  relays  have  no  alternative  but  to  cut 
out  the  last  remaining  transformer  to  save  it. 

POWER-DIRECTIONAL-RELAY  PROTECTION 

It  is  quite  evident  from  the  foregoing  that  a  power-directional 
relay  may  be  installed  in  the  load  side  of  the  transformer  to  give 
adequate  protection.  Excess-current  (overload)  relays  are 
installed  in  the  load  side  as  before.  Now  should  an  internal 
short  develop  in  the  transformer,  the  reversal  of  power  in  the 
secondary  causes  the  power-directional  relay  to  trip  this  side 
of  the  transformer,  thus  relieving  the  overload  on  the  other 
transformers  in  the  bank,  and  then  the  excess-current  relay 
cuts  the  transformer  out  on  the  line  side,  thus  completely 
isolating  it.  In  this  connection,  the  excess-current  relays  give 
protection  against  overload  in  the  line  or  load  past  the  trans- 
former and  power-directional  relays. 

Other  Differential  Methods. — Special  relays,  which  accom- 
plish the  same  protection,  are  sometimes  built,  in  which  the 
actuating  winding  is  wound  in  two  sections.  These  sections 


MOTORS,  TRANSFORMERS,  GENERA  TORS,  AND  LINES    145 

are  connected  to  their  respective  current-transformer  second- 
aries and  so  arranged  that  the  currents  produce  a  bucking  or 


[Wl 


s 

} 

vV 

1 

-r- 

^1 

YA 

^r 
& 

^      <? 

FIG.  140. — Transformer  protection  by  differential  relay. 


FIG.   141. — Diagram  of  double  plunger  relay. 

zero  flux  or  magnetism  on  correct  operation.     When,  however, 

the  current  in  one  winding  exceeds  the  current  in  the  other, 
10 


146 


PROTECTIVE  RELAYS 


the  relay  closes  the  contacts.  Figure  140  shows  a  plunger- 
type  relay  with  a  double  winding,  connected  for  differential 
protection.  A  is  the  large  power  transformer  arranged  with 
series  transformers  B  and  B'  which  supply  the  relay  winding 
D  and  C.  Normally  these  currents  oppose  each  other,  produc- 
ing no  effect  on  the  plunger  E.  But  should  one  current  reverse, 
due  to  a  short  circuit  in  A,  both  assist  in  raising  plunger  E, 
which  closes  contacts  F  and  G,  completing  the  circuit  to  trip 
coils  H  and  7  and  opening  breakers  J  and  K. 


FIG.   142. — Differential  protection  of  two  transformers  on  three-phase  circuit. 

Another  type  of  relay  uses  two  solenoids  operating  a  pivoted 
lever  which  carries  the  contacts.  This  is  shown  diagrammatic- 
ally  in  Fig.  141.  The  two  plungers  E  and  E'  work  under  the 
influence  of  the  solenoids  DC  and  Dr  C".  Normally  current 
from  transformer  Br  in  winding  D  opposes  current  from  trans- 
former B'  in  winding  (7,  resulting  in  no  pull,  while  current  in 
D'  assists  that  in  C',  resulting  in  a  maximum  pull.  Should  the 
current  in  C  and  C"  reverse,  then  plunger  E  is  pulled  up  and 


MOTORS,  TRANSFORMERS,  GENERA  TORS,  AND  LINES    147 

plunger  E'j  losing  its  attraction,  falls  down.  This  moves  the 
lever  L  on  its  axis  M,  closing  contacts  F  and  G  and  tripping 
the  breakers. 

Polyphase  Transformer  Protection. — For  the  sake  of  clarity, 
all  connections  have  been  shown  single-phase.  Two-phase 
requires  two  duplicates  of  the  single-phase.  In  three-phase 
work,  the  transformers  are  generally  worked  in  banks  of  three- 
phase  sets;  that  is,  if  one  transformer  in  a  set  goes  bad,  the 
others  in  that  same  set  go  out  also,  leaving  the  other  sets  in 
the  bank  to  carry  the  load.  Each  three-phase  set  requires  only 
two  relays,  but  requires  three  series  transformers  on  each  side 
of  the  transformer. 

Figure  142  shows  a  typical  three-phase  differential  protec- 
tion connection  for  a  three-phase  set. 


PROTECTING  THREE-PHASE  STAR-DELTA  BANKS 

A  three-phase  bank  of  star-delta  transformers,  having  a 
grounded  neutral,  attempts  to  maintain  the  voltage  equal 
on  all  phases  in  case  of  overload  or  short-circuit.  As  a  result 
in  the  case  of  a  ground  on  the  line,  the  transformers  will  supply 
current  to  the  grounded  wire,  irrespective  of  whether  these 
transformers  are  at  substations  or  generating  stations.  In 
other  words,  if  a  small  bank  of  transformers  is  connected  to 
a  large  system  and  has  its  neutral  grounded,  it  will  be  sub- 
jected to  a  short-circuit  every  time  there  is  a  ground  on  the 
system.  For  this  reason,  small  banks  should  have  their  neutrals 
isolated,  not  only  because  of  the  strain  which  frequent  short- 
circuits  throw  on  them,  but  also  on  account  of  the  frequent 
interruptions  they  entail. 

The  above  argument  applies  principally  to  high-voltage 
systems,  but  it  is  necessary  to  consider  the  same  conditions 
on  low-voltage  four-wire  systems.  Four-wire  systems  are 
generally  used  when  there  is  a  large  amount  of  single-phase 
load  to  be  distributed,  and  as  a  result  the  voltage  on  the  three 
phases  is  liable  to  be  unbalanced.  When  a  bank  of  delta-star 
transformers  is  connected  onto  such  a  system,  the  question 
of  grounding  the  -neutral  must  be  considered  carefully.  As  a 
general  rule,  it  is  unsafe  to  make  such  a  ground  if  the  transformers 


148  PROTECTIVE  RELAYS 

are  small,  but  if  they  are  comparatively  large,  it  may  be  ad- 
visable to  utilize  them  to  assist  in  maintaining  balanced  three- 
phase  voltage.  This  balancing  is  effected  by  drawing  current 
from  the  high-voltage  phase  and  supplying  it  to  the  low-voltage 
phase,  with  the  result  that  there  is  a  flow  of  current  in  the  neutral. 

The  possibility  of  burning  out  the  transformers  can  be  pre- 
vented by  installing  an  overload  relay  in  the  neutral  and  con- 
necting it  so  that  it  will  sound  an  alarm  or  automatically  open 
the  neutral. 

It  frequently  happens  that  star-delta  transformers  are  con- 
nected to  the  main  circuit  through  fuses  and  trouble  is  encoun- 
tered when  a  single  fuse  opens.  If  the  transformers  are  supplying 
a  motor  load  and  the  neutral  is  ungrounded,  the  motor  may 
run  single-phase  and  damage  the  motor;  on  the  other  hand, 
if  the  neutral  is  grounded,  two  of  the  transformers  will  carry 
all  the  load  at  a  much  lower  power  factor  than  normal.  Unless 
there  is  a  means  of  indicating  a  blown  fuse,  the  transformers 
may  carry  the  overload  until  burned  out.  A  relay  installed 
in  the  neutral  and  arranged  to  give  an  alarm  seems  to  be  the  best 
means  of  protection  when  viewed  from  the  various  angles. 

Protection  of  Generators. — Before  the  advent  of  thoroughly 
reliable  reverse-power  relays,  it  was  considered  bad  practice 
to  protect  the  generators  by  overload  relays  because  they  could 
not  be  set  accurately,  and  once  they  started  to  trip,  the  current 
must  be  reduced  very  low  in  order  to  have  them  reset.  The 
undesirability  of  this  is  realized  when  the  momentary  ex- 
change of  power  between  machines,  as  for  instance,  in  syn- 
chronizing, is  considered.  This  rush  is  not  at  all  serious  as  it 
quickly  subsides;  but  if  the  relay  trips  the  breaker  when  there 
is  no  cause  for  it,  then  there  is  a  disadvantage. 

The  main  cause  of  danger  is  in  a  short-circuit  occurring 
in  a  winding.  As  this  short  may  be  only  a  few  turns,  it  might 
quickly  burn  out  a  generator  unless  instantly  detected  and 
isolated.  If  both  ends  of  each  winding  are  accessible  this 
becomes  a  simple  case  of  protection  by  the  differential  method. 
As  shown  in  Fig.  143  the  three  windings  are  connected  in  F,  with 
six  current  transformers  as  shown. 

As  in  the  case  of  transformer  protection,  if  the  same  current 
flows  in  through  one  transformer  and  goes  out  through  the 


MOTORS,  TRANSFORMERS,  GENERA  TORS,  AND  LINES    149 

other,  the  secondary  currents  will  merely  circulate  in  the  wind- 
ings and  will  not  flow  through  the  relay. 

While  this  will  not  protect  against  a  short-circuit  in  any  one 
coil,  yet  if  the  neutral  begrounded,  this  arrangement  will  give 
complete  protection  against  a  ground  on  any  phase  or  a  short- 
circuit  between  phases.  This  is  the  source  of  much  generator 
trouble.  The  burning  of  a  coil  itself  is  usually  not  serious  but 
it  may  be  accompanied  by  a  burning  or  welding  of  the  iron 
laminations,  thus  necessitating  a  complete  dismantling  in  order 
to  replace  a  few  sheets  of  iron. 


_T 


FIG.    143. — Diagram   of   connections   for   differential   protection   of   generators 
using  overload  relays. 


In  protection  of  this  sort,  the  field  circuit  breaker  must  also  be 
tripped  out  as  a  few  short-circuited  turns  may  do  great  damage 
if  running  in  an  excited  field. 

As  trip  connections  have  been  thoroughly  discussed  pre- 
viously they  are  omitted  from  Fig.  143  for  the  sake  of  clarity. 

While  this  method  will  protect  against  damage  by  shorts 
in  the  windings  or  leads,  it  will  not  disconnect  the  generator, 
no  matter  how  severe  the  overload.  It  has  the  objection 
that  it  requires  the  opening  of  the  neutral  point  of  the  generator, 
which  is  often  difficult,  and  it  cannot  be  well  applied  to  delta- 
connected  machines.  Therefore  it  is  often  preferable  to  protect 
against  generator  failure  by  installing  a  high-grade  power- 
directional  relay. 

Protection  by  Power  Directional  Relays. — When  several 
alternators  run  in  parallel  each  one  is  supposed  to  take  its 
share  of  the  load,  but  if  the  prime  mover  should  fail  or  the 
field  circuit  open,  then  the  alternator  may  not  only  refuse 


150  PROTECTIVE  RELAYS 

to  carry  its  share  of  the  load,  but  may  become  motored.  If 
the  refusal  to  carry  load  is  due  to  a  short-circuit  in  the  wind- 
ing it  may  also  cause  a  reversal  of  power  which  will  intensify  the 
short-circuit.  To  guard  against  this,  the  power-directional 
(reverse-power  or  reverse-overload)  relays  may  be  installed 
between  the  generator  and  bus.  These  relays  should  be  set 
with  a  time  setting  of  1  or  2  sec.  on  the  excess-current  element 
as  there  are  often  heavy  surges  of  current  which  may  flow 
between  machines  due  to  line  switching  or  synchronizing  and  a 
time  setting  which  is  too  short  will  cause  unnecessary  tripping. 

Even  when  the  machines  are  protected  in  this  manner,  every 
generator  or  prime  mover  should  be  provided  with  an  over-speed 
device  which  prevents  the  machines  from  speeding,  as  occa- 
sions may  occur  where  neither  excess-current  differential  con- 
nections nor  power-directional  relays  will  give  the  proper  protec- 
tion against  this. 

Protection  of  Single  Lines. — While  transmission  lines  are 
seldom  damaged  by  overloads,  except  by  an  arc  at  one  par- 
ticular point,  yet  the  great  majority  of  disturbances  originate  on 
the  lines  owing  to  short-circuits  caused  by  the  lines  themselves 
falling  or  becoming  grounded,  or  arcs  starting  from  something 
shorting  the  wires,  as  for  instance  a  crosswire  dropping  across 
the  lines.  Therefore  it  becomes  necessary  to  disconnect  a  faulty 
line  very  quickly  in  case  of  short-circuit.  To  do  this,  overload 
relays  are  installed  in  each  line  to  be  protected.  Even  on  a 
single-phase  line,  two  transformers  should  be  employed  as  a 
double  ground  on  opposite  lines  may  occur  at  such  points 
that  they  cause  a  short-circuit  in  which  the  protective  trans- 
former is  cut  out.  The  connections  are  similar  to  those  de- 
scribed for  the  induction  motor  in  Fig.  130  and  need  not  be 
duplicated.  Additional  schemes  for  line  protection  are  dis- 
cussed under  the  pilot-wire  and  split-conductor  systems. 

Protection  Against  Grounds. — If  a  three-phase  line  is  grounded 
solid  at  the  neutral,  then  adequate  protection  is  obtained  by 
the  usual  line  relays.  Solid  grounding  however,  has  been 
shown  to  be  undesirable  for  other  reasons  in  some  cases,  and 
therefore  the  neutral  is  grounded  through  a  current-limiting 
resistor.  In  such  cases  it  becomes  necessary  to  provide  addi- 
tional protection  against  line  grounds,  and  this  is  accomplished 


MOTORS,  TRANSFORMERS,  GENERATORS,  AND  LINES    151 

by  inserting  a  relay  in  the  neutral  connection  of  the  line  relays. 
This  relay  is  then  actuated  by  any  unbalanced  current  which 
may  flow  in  the  neutral. 

This  relay  may  be  a  plain-current  relay  set  to  operate  on  a 
low  value  of  current  as  in  the  case  of  a  system  with  the  neutral 
grounded  through  a  comparatively  low  resistor.  Or  it  may 
be  a  watt  relay  used  with  the  connections  shown  in  Fig.  144. 


GROUND  PROTECTION  USING  WATT  RELAY 


FEEDER 


METERS 
OVERLOAD 
fitLAYS.  £TC. 


FIG.   144. — Ground  protection  using  a  watt  relay. 

The  current  coil  is  inserted  in  the  neutral  between  the  trans- 
formers and  line  relays,  while  the  potential  coil  is  energized  by 
the  drop  in  voltage  across  a  resistor  in  the  neutral  of  the  poten- 
tial transformers. 

This  relay  is  set  to  operate  at  a  very  low  value  of  about  1 
amp.  or  less.  It  has  an  adjustable  time  element  and  can  there- 
fore be  set  to  select  between  successive  sections  of  line. 


CHAPTER    XII 
PROTECTION    OF    PARALLEL    FEEDERS 

When  substations  are  tied  into  the  main  generating  station 
or  when  important  industrial  concerns  are  to  be  supplied  with 
uninterrupted  service,  reliance  is  not  placed  in  one  single  trans- 
mission line.  Generally  there  are  a  number  of  lines  run  and 
connected  in  parallel.  These  lines  may  or  may  not  run  on  the 
same  poles  or  the  same  right-of-way.  Then  in  the  event  of 
a  crippled  line,  protective  relays  are  provided  to  discriminate 
immediately  and  isolate  the  defective  line  while  the  other  lines 
instantly  assume  the  increased  load  without  a  second's  inter- 
ruption. The  following  report,  taken  from  the  Thirty-fifth 
Annual  Convention  Proceedings  of  the  A.  I.  E.  E.,  shows  the 
vast  saving  which  may  be  expected  from  the  use  of  properly 
protected  parallel  feeders: 

SAVINGS  EFFECTED  BY  A  PARALLEL  OPERATION 
OF  FEEDERS 

"The  savings  which  can  be  effected  by  a  parallel  operation  of  feeders 
depend  in  a  large  degree  upon  the  design  of  the  transmission  system  and 
the  ratio  of  the  capacity  of  the  line  to  the  capacity  of  the  individual 
synchronous  converters  or  transformers  which  are  supplied  by  this 
feeder.  In  one  system  supplying  converters  varying  from  500  to 
4,000  kw.  in  size  together  with  step-down  transformers  in  substations 
of  1,500  and  3,000  kw.  capacity  and  also  industrial  substations  on  the 
premises  of  customers  ranging  from  about  500  to  4,000  kw.  in  capacity, 
it  was  estimated  that  if  the  feeders  could  be  operated  in  parallel  then  a 
saving  of  20  per  cent  could  be  made  in  the  amount  of  investment. 
As  the  installation  in  question  had  a  book  value  of  about  $5,000,000, 
there  was  a  possible  saving  estimated  at  $1,000,000.  This  company 
has  been  operating  feeders  in  multiple  for  about  two  years.  During  this 
period  it  has  realized  nearly  40  per  cent  of  the  possible  saving  in  the 
investment  in  feeders,  and  this  has  been  secured  by  an  actual  reduction 
in  the  number  of  feeders,  notwithstanding  a  considerable  increase  in 
the  maximum  load. 

152 


PROTECTION  OF  PARALLEL  FEEDERS        153 

* '  The  same  company  also  reports  in  the  four  years  preceding  the  installa- 
tion of  the  relays  permitting  parallel  operation  of  feeders  that  they 
averaged  27  burnouts  per  year  which  could  not  be  definitely  ascribed 
to  external  causes,  while  in  the  two  years  since  the  feeders  have  been 
operated  in  parallel,  the  corresponding  figure  was  16  burnouts.  This 
would  indicate  a  reduction  of  40  per  cent  in  the  burnouts  of  cables 
due  to  operation  of  the  feeders  in  multiple,  although  the  cables  were 
actually  more  heavily  loaded.  While  the  time  is  rather  too  short  to 
accept  these  figures  as  final  and  conclusive,  it  apparently  indicates 
that  the  operation  of  feeders  in  parallel  reduces  the  number  of  troubles 
due  to  internal  causes." 


VARIOUS  METHODS  EMPLOYED 

There  are  six  methods  commonly  employed  to  protect  parallel 
tie  lines  or  feeders:  . 

1.  Inverse-time-limit  relay  discrimination. 

2.  The  balanced-protection  system. 

3.  The  split-conductor  system. 

4.  The  pilot-wire  system. 

5.  The  power-directional  relay  system. 

6.  The  differential-relay  system. 

Of  these  six  methods,  four  of  them  employ  simple  excess 
current  (overload)  relays,  the  efficiency  of  the  protection  depend- 
ing largely  upon  the  grade  of  relay  employed.  The  sixth 
system  employs  a  specially  designed  current-differential  relay 
as  the  name  implies,  while  the  fifth  system,  which  is  one  of  the 
most  reliable  and  efficient,  employs  a  power  directional  relay. 

Inverse-time-limit-relay  Discrimination. — About  the  first 
attempt  to  protect  parallel  feeders  was  made  by  installing 
plain-overload  or  excess-current  relays  with  an  inverse-time  delay, 
such  as  the  bellows  or  induction  type,  in  each  line  and  depending 
on  the  inherent  division  of  overload  current  with  the  consequent 
variation  in  relay-time  delay  to  discriminate  and  isolate  the 
defective  feeder.  This  system  is  quite  satisfactory  where  an 
occasional  interruption  is  not  of  serious  consequences,  and  where 
there  are  a  number  of  parallel  feeders,  its  selectivity  increasing 
with  the  number  of  lines. 


154 


PROTECTIVE  RELAYS 


As  an  example  of  how  inverse-time-limit  relays  may  dis- 
criminate, consider  the  parallel  tie  lines  in  Fig.  145.  The  genera- 
tor bus  is  tied  to  the  substation  bus  with  five  three-phase 
transmission  lines,  A,  B,  C,  D  and  J57,and  these  lines  are  equipped 
with  circuit  breakers  and  relays  at  both  ends. 

Now  suppose  a  short-circuit  occurs  at  X.  A  rush  of  current 
flows  directly  from  the  generating  bus.  Say  for  example  it 
amounts  to  10,000  amp.  Another  rush  occurs  over  the  lines 
A,  C,  D  and  E  and  through  the  sub-bus,  back  to  the  short 
at  X.  Say  this  amounts  to  6,000  amp.  This  is  divided  between 


CIRCUIT 


FIG.  145. — Elementary  diagram  of  parallel  feeder  protection  by  inverse  time 

discrimination. 


the  four  feeders  A,  C,  D  and  E,  each  taking  one-fourth  so  the 
current  in  each  is  1,500  amp.  It  is  very  evident  that  if  all  the 
relays  are  set  equally,  the  one  having  10,000  amp.  will  go  out 
much  quicker  than  the  ones  having  1,500  amp.  Likewise 
the  one  having  6,000  (B  from  sub-bus)  will  go  out  quicker  than 
the  ones  having  only  1,500  amp. 

The  greater  the  number  of  lines,  the  greater  the  differences 
of  current  during  a  short-circuit,  and  the  more  protection. 

Suppose,  however,  it  is  not  a  "dead"  short  but  merely  a 
high-resistance  short,  making  only  a  moderate  overload.  This 
throws  the  action  on  an  entirely  different  part  of  the  time 
curve,  and  the  relays  may  or  may  not  distinguish  which  is  the 
defective  feeder. 

It  is  very  evident  that  the  selective  action  cannot  be  obtained 
from  150  per  cent  overload  to  a  "dead"  short,  and  since  this 
can  be  done  with  power-directional  relays,  the  selective  inverse- 
time-limit-relay  system  is  only  employed  where  low  initial 


PROTECTION  OF  PARALLEL  FEEDERS 


155 


expense  is  of  paramount  importance  and  absolute  continuity 
of  service  a  secondary  consideration. 

The  Balanced  Protection  System. — When  there  are  a  com- 
paratively large  number  of  parallel  feeders  which  may  be  divided 
into  pairs,  quite  efficient  protection  may  be  obtained  by  using 
a  simple  excess-current  (overload)  relay,  connected  differen- 
tially to  the  two  lines  much  in  the  same  manner  as  that  described 
under  differential-transformer  protection.  In  this  case,  if 

STATION  A  STATION  B 


STATION  A 


STATION  B 


STATION  A 


STATION  B 


FIG.   146. — Three   views   of    balanced   protection  of   two   parallel   lines.     (Not 

discriminating.) 

trouble  occurs  in  one  feeder,  then  both  feeders  in  the  pair  go  out. 
If,  however,  both  feeders  develop  trouble  at  once,  as  might  be 
the  case  if  both  sets  were  carried  on  the  same  pole  line,  then 
neither  set  would  go  out  unless  equipped  with  suitable  reactors; 
or  if  equipped  with  plain  overload  relays,  the  whole  substation 
might  go  out  unless  there  were  enough  lines  to  enable  the  inverse- 
time-limit  relays  to  trip  out  the  bad  feeder  by  their  selective 
time  delay.  Consider  Fig.  146.  The  transformer  on  the  first 
feeder  is  connected  with  its  secondary  in  series  with  that  of 
the  other  transformer.  Normally  the  currents  are  as  shown 
by  the  arrows  and  no  current  flows  through  the  relay.  Then 
consider  the  second  figure.  A  short-circuit  has  occurred  at  -X". 


156  PROTECTIVE  RELAYS 

There  is  an  overload  on  both  lines,  but  the  current  has 
reversed  as  shown;  it  now  bucks  the  opposite  transformer 
with  the  result  that  the  two  currents  from  both  flow  through 
the  relay,  which  closes  and  trips  out  both  breakers  at  station 
B.  This  relieves  the  overload  on  the  good  feeder,  but  the  over- 
load still  exists  on  the  shorted  feeder.  Consequently,  there 
is  again  an  unbalancing  of  current  in  the  current  transformers 
at  the  station  A,  and  this  unbalancing  causes  the  relay  at  station 
A  to  trip  out  both  feeders  at  station  A,  thereby  isolating  both 
feeders  at  both  ends.  An  open  circuit  in  either  line  will  also 
cause  an  unbalancing  and  tripping  of  relays. 

It  will  be  noted  that  both  substation  breakers  are  tripped  no 
matter  on  which  feeder  the  trouble  occurred,  also  that  if  a  simul- 
taneous short  occurred  on  both  feeders,  neither  breaker  would 
trip  as  the  current  in  both  transformers  would  reverse,  giving 
the  same  effect  as  normal  direction. 

This  can  be  overcome  by  correctly  inserting  reactances 
in  the  substation  end  in  one  feeder  and  in  the  generator  end 
of  the  other  feeder,  as  will  be  more  fully  explained  under  the 
"Split-conductor  System." 

After  the  tripping  of  a  pair  of  lines,  it  is  the  customary  prac- 
tice to  locate  the  good  line  and  put  it  back  in  service  with  straight 
overload  protection,  in  place  of  the  differential  protection. 

In  order  to  protect  against  short-circuits  on  the  substation 
bus  itself,  or  against  any  possibility  which  would  not  cause  an 
unbalancing  of  currents,  or  a  relative  reversal  of  current  on 
a  group  of  parallel  feeders,  the  generator  end  of  all  feeders 
should  be  provided  with  inverse-time-limit  relays  having  com- 
paratively high "  settings.  With  this  arrangement,  it  often 
happens  that  if  the  fault  occurs  on  the  line  between  the  main 
and  substation,  the  substation  relay  clears  the  fault  very 
quickly  at  the  substation  end,  so  that  the  inverse-time-limit 
relay  on  the  faulty  line  only  at  the  generator  end  trips,  thus 
leaving  the  good  line  of  the  pair  connected  at  the  main  station. 
Then  if  a  voltmeter  is  connected  at  the  substation  to  ascertain 
which  line  is  alive,  the  good  line  may  be  quickly  put  back  in 
service. 

Differential-balance-relay  Protection. — In  this  system,  it 
is  necessary  to  have  a  large  number  of  parallel  feeders  or  tie 


PROTECTION  OF  PARALLEL  FEEDERS 


157 


lines  in  order  to  secure  an  unbalanced  current  in  the  event  of 
a  short  on  any  line.  A  relay  connected  as  shown  in  Fig.  147 
consists  of  two  solenoids  A  and  B  each  pulling  down  on  plungers 
C  and  D  which  are  attached  to  the  rocker  arm  E.  This  rocker 


FIQ.  147. — Elementary    diagram    of    parallel    line    protection    by    differential 

overload  relays. 


arm  carries  a  moving  contact  F  which  may  make  contact  with 
either  G  or  //  according  to  which  solenoid  exerts  the  stronger  pull. 
The  feeders  are  arranged  in  pairs  with  the  current  trans- 
former of  one  line  connected  to  solenoid  A  and  the  current 
transformer  of  the  other  line  connected  to  solenoid  B.  As 
long  as  both  lines  carry  their  share  of  the  load,  the  pull  on  both 
plungers  is  equal  no  matter  what  the  magnitude  of  the  load, 


FIG.   148. — Short-circuit  on  a  feeder  protected  by  differential  overload  relays. 


but  should  a  short  occur  as  at  X  in  Fig.  148  then  the  current 
is  no  longer  equal,  thus  allowing  one  solenoid  to  overpower  the 
other  and  trip  the  breaker  on  the  defective  line.  These  relays 
may  be  used  at  the  generating  end  and  at  the  receiving  ends 
of  the  line  if  there  are  three  or  more  lines  entering.  Complete 


158 


PROTECTIVE  RELAYS 


diagrams  of  instantaneous  balance  relays  and  time  delay  relays 
are  shown  in  Fig.  149. 

Split-conductor  System. — This  method  utilizes  a  cable  with 
two  wires  in  parallel  instead  of  a  single  wire.  A  two-phase 
cable  would  thus  have  eight  wires,  and  a  three-phase  cable 


4-C.Bus 


Aux/tiary  Sw/tches 
Open  when  OH  Cir- 
cuit Breaker  Opens 


_,  Current ••"* 
tfronsformir  \ 


Current 
Transformer 


FIG.   149. — Connections  of  G.  E.  overload  relays  and  balanced  current  relays 
for  protection  of  parallel  lines. 

six  wires.  Figure  150  shows  the  two  wires  of  one-phase  of  a  six- 
wire  cable.  The  other  two  sets  would  be  connected  in  a  similar 
manner.  A  differential-current  transformer  is  used  having 
two  primaries  which  buck  each  other  on  balanced  current  and, 
therefore,  produce  no  current  in  the  relay  during  normal  opera- 
tion. Suppose  a  defect  occurred  between  the  two  lines  as  in 


PROTECTION  OF  PARALLEL  FEEDERS 


159 


Fig.  151.  The  currents  would  no  longer  divide  equally  but  less 
would  flow  in  one  line  than  the  other.  This  unbalancing  of 
current  would  excite  the  relays  and  open  the  breakers  at  both 


STATION.  A 


STATION  B 


HEALTHY  CONDITION 

^r ARROWS  INDICATE.  RELATIVE  Dmec/nons 
AND  UNIT  iNTEtismta  of  CURRENT  FLOW 

FIG.  150. — Schematic  diagram  of  connections  of  the  split  conductor  system. 

ends,   effectually  isolating  the   cable.     Figure   152   shows  the 
conditions  of  a  low  resistance  fault. 

However,    a  short  only  between  two  sections  of  the  same 
line  would  cause  no  damage;  but  such  a  short  seldom  occurs. 


STATION  A 


STATION.  B 


FIG.   151. — Direction  of  power  flow  with  "short." 

Generally  one  pair  short-circuits  to  another  pair.  However, 
if  one  pair  shorted  to  either  wire  of  another  pair,  the  currents 
in  the  two  conductors  would  be  unequal  and  the  breakers  at 
both  ends  would  trip  out.  If,  however,  the  two  sections  of 


STATION  A 


STATION  B 


low  RESISTANCE. 

FAULT  CONDITION 

FIG.   152. — Length  of  arrows  represent  the  intensity  of  the  power  through  the 

fault. 

the  same  line  shorted  together  and  these  shorted  to  two  similar 
sections,  the  currents  would  still  be  balanced  and  no  protection 
result.  This  is  overcome  by  installing  a  reactance  in  the  lines 
as  shown  in  Fig.  153.  It  will  be  noticed  that  a  reactor  is  in  one 


160 


PROTECTIVE  RELAYS 


side  of  the  split  at  one  end  of  the  cable  and  a  reactor  is  in  the 
opposite  split  at  the  opposite  end.  Now,  if  both  splits  short- 
circuit,  the  current  is  no  longer  balanced,  due  to  the  reactor, 
and  the  relays  trip  out  the  breakers.  For  further  details  see  the 
paper  by  W.  H.  Cole,  on  "  Split-Conductor  Cables — Balanced 
Protection,"  Proceedings  A.  I.  E.  E.,  July,  1918,  p.  793. 

A  short-circuit  on  one  cable  naturally  overloads  all  the  cables, 
but  if  the  other  cables  are  intact  the  current  will  divide  equally 
in  the  split  conductors  and  the  relays  will  not  be  energized, 
no  matter  what  the  overload.  As  in  the  previous  example 
this  is  excellent  protection  but  is  very  expensive,  requiring 


DIFFERENTIAL  CURRENT 
•  TRANSFORMER 

.-  Reactor 


Reactor..  ->• 


To  Relay  To  Relay 

FIG.   153. — Showing  the  use  of  reactors  in  the  split  conductor  system. 

reactors  and  large  cables.  The  same  protection  is  given  by 
using  power-directional  relays  (to  be  described  later)  and  this 
has  discouraged  the  extensive  use  of  the  split-conductor  system. 

The  Pilot-wire  System. — As  has  already  been  shown,  differen- 
tial relays  are  practically  indispensable  in  protecting  trans- 
former banks  by  instantly  cutting  out  a  defective  unit  without 
interrupting  the  service.  Closely  allied  with  this  protection 
is  the  differential  protection  of  parallel  feeders,  called  the  pilot- 
wire  system. 

Referring  to  Fig.  154  the  main  generating  station  is  tied 
into  the  substation  by  means  of  several  feeders.  Each  feeder  is 
equipped  with  circuit  breaker,  current  transformers  and  relays 
at  each  end.  Parallel  to  the  feeders  are  two  small  wires,  gen- 
erally about  No.  10  B.  &  S.  gage,  connecting  the  two  trans- 
former secondaries  and  relays  in  series.  Under  normal  con- 
ditions, no  current  will  flow  in  the  pilot  wires  as  the  secondary 
windings  are  arranged  to  "buck"  each  other,  resulting  in  zero 
current,  regardless  of  load.  The  direction  of  the  current  is  as 


PROTECTION  OF  PARALLEL  FEEDERS 


161 


shown  by  the  arrows.  But  suppose  a  "short"  or  other  defect 
occurred  between  the  two  stations  as  at  X.  Power  starts  to  feed 
into  the  short  from  the  generator  bus,  and,  in  addition,  it  feeds 
over  the  good  feeders,  through  the  substation  bus  and  back  into 
the  short-circuit. 

Thus,  there  is  a  heavy  excess  current  on  every  current  trans- 
former, A  and  B  and  G  and  H.  Since  the  current  is  normal 
in  the  good  feeder,  the  transformer  secondaries  buck  and  no 


MAIN 


SUB 


FIG.  154. — Arrows   indicate  direction  of   power  flow  with  'short'  on  a  pilot- wire 

protected  system. 

current  results  in  the  pilots,  no  matter  how  heavy  the  excess 
current.  The  current  in  A  is  still  in  the  same  direction,  but 
the  current  in  transformer  B  has  reversed.  Therefore,  it 
no  longer  opposes  A,  but  the  two  transformers  unite  in  forcing 
a  current  over  the  pilot  wires  and  through  the  relays,  which 
instantly  closes  their  contacts,  tripping  both  breaker  I  and  J, 
thus  effectually  isolating  the  defective  feeder,  relieving  excess 
current  on  the  good  feeders  and  continuing  service  without  a 
second's  interruption. 

If  desired,  instead  of  an  instantaneous  trip,  relays  may  be 
used  which  have  a  time  delay  inversely  proportional  to  the 
severity  of  the  disturbance. 

Sometimes  the  feeders  are  also  equipped  with  plain  over- 
load relays  in  addition  to  the  differential  relays,  but  in  that  case, 
the  time  must  always  be  set  longer  than  that  of  the  differen- 
11 


162  PROTECTIVE  RELAYS 

tial  relays,  thus  enabling  them  to  distinguish  between  a  short 
in  a  feeder  and  an  overload  in  or  past  the  substation. 

This  method  of  protection,  although  widely  used  in  Europe, 
has  not  met  with  much  favor  in  this  country,  except  on  small 
systems  where  the  substations  are  close  together,  as  the  cost 
of  the  pilot  wires  is  quite  high;  besides,  the  same  protection 
is  afforded  by  power-directional  relays,  which  will  be  described 
later. 

Another  disadvantage  of  the  differential  method  of  protec- 
tion is  that  anything  which  may  damage  the  transmission  line 
would  be  very  liable  to  damage  the  pilot  wires,  and  it  is  very 
evident  that  if  they  become  open-circuited,  the  relays  can- 
not operate  no  matter  how  severe  the  disturbance.  Then  if 
the  pilot  wires  become  shorted,  even  though  the  feeders  are 
intact,  the  breakers  will  open. 

To  this  is  added  the  great  danger  that  a  high-tension  feeder 
in  falling,  or  breaking,  may  catch  onto  the  pilot  and  introduce 
a  dangerous  potential  into  the  station. 

Pilot-wire  systems  frequently  make  use  of  current  trans- 
formers with  a  normal  secondary  current  of  J-£  amp.  with 
relays  wound  correspondingly.  Thus,  if  a  300  to  J-^  amp.  trans- 
former were  used,  and  the  relay  set  for  Ji  amp.,  the  system  would 
clear  a  ground  drawing  as  low  as  150  amp.  It  has  been  cal- 
culated that  if  a  No.  10  pilot  wire  is  used,  these  relays  can  be 
operated  successfully  where  the  stations  are  between  2  and  3 
miles  apart,  the  maximum  allowable  distance  depending  upon 
the  setting  of  the  relay,  which  in  turn  is  determined  by  the  mini- 
mum ground  current  which  is  expected  to  flow  during  trouble. 
Further  information  on  this  subject  is  given  in  a  paper  by 
R.  F.  SCHUCHARDT  on  "Protective  Relays/'  Transactions 
A.  I.  E.  E.,  Vol.  XXXVI,  p.  383,  1917. 

Sometimes  a  loop  or  ring  system  contains  so  many  sub- 
stations that  the  time  intervals,  which  it  is  necessary  to  allow 
between  relays  in  series,  add  up  to  an  unsafe  amount  on  the 
relays  at  the  generating  station.  If  the  loop  covers  a  small  terri- 
tory, in  may  be  convenient  to  install  pilot  wires  between  some  of 
the  substations.  The  connections  are  as  shown  in  Fig.  155  and  it 
will  be  observed  that  under  normal  conditions  the  current 
transformers  at  each  end  of  a  conductor  are  short-circuited 


PROTECTION  OF  PARALLEL  FEEDERS 


163 


upon  each  other  through  the  pilot  wire.  However,  the  pilot 
wire  may  have  sufficient  resistance  so  that  the  current  will 
divide  and  part  of  it  leak  through  the  relays  at  each  end.  There- 
fore, current  transformers  are  used  which  have  a  secondary 
rating  of  J£  or  sometimes  1  amp.,  which  decreases  the  potential 
drop  between  the  two  ends  of  the  pilot  wire  and  at  the  same 


SUBSTATION  BUS  BARS 


-  1 

[6     66       CIRCUIT 
\    Q     Q     Q       BREAKER 

4lll|)L, 

•""Hip 

CO.  RELAYS 
OF  LOW 
CURRENT 
RANGE      ' 

L 

Of 
pi 

t 

I 

"( 

\J 

o 

1    1 

3  OR  4  PILOT- 
WIRES    •-- 
REQUIRED 


SPECIAL  CURRENT 
>  TRANSFORMERS  HA  VI NG 
> SMALL  SECONDARY 

CURRENT 


SUBSTATION  BUS  BARS 


FIG.  155.  —  Pilot  wire  system  arranged  so  that  secondary  current  normally  circu- 
lates between  stations. 


time  requires  the  use  of  a  relay  which  operates  on  a  smaller 
current  and  consequently  has  a  higher  impedance  so  that 
less  current  will  leak  through  it. 

When  a  short-circuit  or  ground  occurs  on  the  line  between 
the  two  substations,  the  current  transformers  at  the  two  ends 
are  no  longer  short-circuited  upon  each  other,  but  the  currents 
which  they  produce  are  opposed  to  each  other  so  that  current 
must  flow  through  the  relays  and  trip  out  the  circuit  breakers 
at  each  end. 


164 


PROTECTIVE  RELAYS 


PARALLEL -FEEDER  PROTECTION  BY  POWER-DIRECTIONAL 

RELAYS 

This  system  of  protection,  which  is  used  extensively  on  radial- 
distribution  systems,  utilizes  excess-current  relays  of  the  induc- 
tion type  at  the  generating  end  of  the  transmission  lines  and 
power-directional  relays  at  the  substation  or  receiving  end  of 
the  line.  Referring  to  Fig.  156  the  generator  bus  G  is  tied 
to  the  substation  bus  S  by  three  parallel  three-phase  lines 
A,  B  and  C.  The  excess-current  relays  and  circuit  breakers 


-I/^ 


^I^k 


$& 


_i^r\ 


±&^ 


Fio.  156.- 


-Elementary   use   of   power   directional   relays   on   parallel    feeders 
Trip  and  potential  connections  omitted. 


are  shown  at  E  and  the  power-directional  relays  are  shown 
at  R.  Now  assume  a  short  at  X.  As  has  been  previously 
shown,  a  heavy  current  flows  through  the  relay  on  line  B,  and 
also,  but  of  less  magnitude,  on  the  lines  A  and  C.  But  the 
current  in  relay  R  on  line  B  has  reversed  with  respect  to  the 
bus  voltage.  Consequently  this  relay  trips  out  the  substation 
end  of  this  line,  relieving  the  excess  current  on  lines  A  and  C. 
The  overload  still  continues  on  line  B  until  excess-current 
relay  E  trips  out  the  breaker  on  the  generating  end,  thus  com- 
pletely isolating  the  defective  feeder.  Figures  157a  and  1576 
show  the  typical  connections  for  power-directional  relays 


PROTECTION  OF  PARALLEL  FEEDERS 


165 


on  a  three-phase  circuit.  Figure  157 a  shows  a  connection  with 
the  current  in  each  relay  leading  the  voltage  by  30  deg.  at  100 
per  cent  line  power  factor,  and  Fig.  1576  shows  a  connection  with 
the  current  and  voltage  in  any  relay  in  phase  with  each  other. 
Figures  158,  159,  and  160  give  typical  diagrams  furnished  by  the 
Westinghouse  Co.  and  the  General  Electric  Co. 


Voltage  Trans. 


(B) 

A  -Y  TRANSFORMERS  BE7WEEN 
LINES  AMD  STATION  BUS  BARS 


FIG.  157a. — Connections  for  Westinghouse  power  directional  relays  with 
current  leading  the  voltage  in  any  relay  by  30  degrees. 

FIG.  157b. — In  this  diagram,  the  current  and  voltage  are  in  phase  in  each  relay 
at  100  per  cent  line  power  factor. 


Although  this  is  a  simple  system,  yet  the  development  of 
a  reliable  power-directional  relay  which  would  function  cor- 
rectly was  by  no  means  a  simple  task.  For  further  information 
on  this,  the  reader  is  referred  to  the  chapters  on  "  Power-Direc- 
tional Relays"  and  "Characteristics  of  Electrical  Disturbances.7' 

Cross-connected  Power-directional  Relays. — The  use  of 
cross-connected  relays  is  one  of  the  most  desirable  means  of 
securing  discriminating  protection  under  certain  circumstances. 
It  also  finds  ready  application  on  many  systems  where  the 
feeders  are  run  in  parallel  between  switching  points  where 
conditions  are  so  complex  that  selective  timing  becomes  impos- 
sible. The  cross-connected  system  is  practically  instantaneous 


166 


PROTECTIVE  RELAYS 


in  operation  if  desired,  but  it  may  be  provided  with  any  time 
delay  desired  by  means  of  the  setting  on  the  excess-current 
members  of  the  power-directional  relays. 


Another  advantage  of  the  cross-connected  system  is  that 
the  relays  may  be  set  to  clear  a  fault  which  draws  less  than 
full-load  current  on  each  feeder.  This  enables  the  correct 
clearing  of  trouble  on  a  system  having  the  neutral  grounded 


PROTECTION  OF  PARALLEL  FEEDERS 


167 


through  such  a  high  resistance  that  the  total  load  and  trouble 
current  may  be  less  than  the  maximum  load  current  of  that  cable. 
If  the  parallel  feeders  happen  to  be  on  a  system  with  more 
than  one  generator  station,  there  may  be  occasions  when  the 
power  flow  is  first  one  way  and  then  in  the  opposite  direction. 
With  the  ordinary  connections,  this  often  necessitates  a  change 


on 

Circuit 
Breaker 


Fio.  159. — Connections  of  G.  E.  polyphase  power  directional  relay  and  overload 
relays  for  protecting  a  three-phase  system  with  ungrounded  neutral. 

in  the  relay  settings,  but  with  the  cross-connected  system, 
the  need  for  this  change  is  obviated,  as  the  adjustment  is  the 
same  regardless  of  the  direction  of  power  flow. 

Some  manufacturers  and  users  claim  that  the  cross-connected 
system  is  more  economical  than  the  split-conductor  or  the 
pilot-wire  system,  because  it  does  not  require  extra  cables. 
They  also  claim  it  to  be  superior  to  the  balanced  protection 


168 


PROTECTIVE  RELAYS 


using  two-in-pair,  because  the  cross-connected  system  cuts 
out  only  the  defective  feeder,  while  the  balanced  system  cuts  out 
both  feeders,  and  the  good  one  must  be  located  in  some  other 
manner  before  it  can  be  put  back  in  service. 

The  schematic  diagram  of  Fig.   161  shows  the  connections 
of   cross-connected  reverse-power  relays  applied  to  a  system 


FIG.  160. — Connections  for  G.  E.  polyphase  power  directional  relay  and  over- 
load relays  for  protecting  a  three-phase  grounded  neutral  circuit. 

consisting  of  a  generating  station  and  a  substation  connected 
by  two  parallel  feeders.  To  simplify  it,  the  diagram  shows 
only  one  phase  of  each  feeder.  A  complete  diagram  for  a  pair 
of  three-phase  feeders  is  shown  in  Fig.  162.  Here,  however, 
the  tripping  circuit  is  omitted. 

It  should  be  borne  in  mind  that,  while  shown  in  Figs.  161 
and  162  for  a  comparatively  simple  condition,  this  scheme 
can  be  used  with  equal  success  in  any  part  of  a  complicated 
network.  While,  preferably,  the  cables  in  the  parallel  system 


PROTECTION  OF  PARALLEL  FEEDERS 


169 


should  be  alike,  if  there  is  a  difference  in  their  impedance  this 
difference  can  be  compensated  for  by  simple  means. 

Under  normal  conditions,  the  load  in  each  of  the  cables  will 


GENERATING  STATION 


FIG.  161. — Schematic  diagram  of  Westinghouse  cross-connected  relays  with 
connections  for  one  phase  only  shown.  Voltage  and  trip  circuits  omitted  for 
clarity.  Arrows  show  direction  of  power  flow  with  "  short "  on  right-hand  feeder. 

be  the  same  and,  since  the  relays  have  a  higher  impedance  than 
the  current  transformers,  the  current  from  the  latter  will,  there- 
fore, circulate  through  all  of  them  in  series  without  any  flowing 


Three-PhaseBus. 


for  open  posit/on  of  for •  closed position  of  c  B  A 
Oil  Breaker.  Oil  Breaker   • 

FIG.   162. — Connections  similar  to  Fig.   161  but  with  trip  circuit  connections 

omitted. 

through  the  relays.  If  the  trouble  occurs  at  any  point  outside 
the  section  protected  by  these  cross-connected  relays,  the 
current  through  the  cables  will  still  be  balanced  and  conse- 


170 


PROTECTIVE  RELAYS 


quently  there  is  no  force  tending  to  operate  the  relays.  On 
the  other  hand,  if  trouble  occurs  on  a  cable  within  the  section, 
the  current  through  the  defective  cable  will  be  higher  than  that 
in  the  others  and  the  excess  current  from  its  current  transformers 
must,  therefore,  pass  through  the  relays.  While,  under  this 
unbalanced  condition,  current  will  flow  through  all  the  relays, 
it  will  be  observed  that  the  current  is  in  the  proper  direction 
to  cause  the  relay  to  act  only  in  the  relays  at  each  end  of  the 
defective  cables. 

In  Fig.  162  are  shown  pallet  switches  connected  in  the  trans- 
former secondary  circuit.  These  are  also  connected  mechan- 
ically to  the  operating  mechanism  of  the  breaker  so  that  when 
the  breaker  opens  the  current  transformers  on  the  feeder  con- 
trolled will  be  short-circuited.  By  this  method  a  cable  can 
be  cut  out  of  service  without  interfering  with  the  electrical 
balance  in  the  current-transformer  circuit. 

6, 


CURRENT' 
7RAHS.  • 


FIG.  163. — Connections  of  Westinghouse  double  contact  relay  (shown  for  one 

phase  only). 

Differential-power-directional    or    Double -contact  Relays. — 

It  will  be  noticed  that  when  the  cross-connected  relays  are  applied 
to  two  parallel  feeders,  they  are  actually  in  parallel,  and  one  closes 
with  power  in  one  direction  and  one  with  power  in  the  other. 

Therefore,  instead  of  using  two  relays/  it  is  possible  to  put 
two  contacts  on  the  upper  element  to  perform  the  same  duties. 
Thus,  if  the  disk  turns  to  the  right,  it  closes  one  contact  and 
if  it  turns  to  the  left,  it  closes  the  other  contact. 

This  is  clearly  shown  in  Fig.   163,  which  shows  two  lines, 


PROTECTION  OF  PARALLEL  FEEDERS        171 

feeding  from  or  to  the  bus  C.  The  transformers  are  differen- 
tially connected,  i.e.,  if  the  load  is  divided  normally,  the  current 
merely  circulates  through  the  transformer  secondaries  and 
will  not  pass  through  the  relay,  as  there  is  as  much  tendency 
to  flow  one  way  as  the  other. 

But  should  one  current  reverse,  due  to  a  short-circuit,  there  is 
an  immediate  flow  of  current  in  the  relay  and  its  direction  is  such 
that  it  causes  the  upper  disk  to  turn  in  the  proper  direction 
to  close  the  contacts,  which  trips  the  breaker  on  the  defective 
line.  This  is  true  whether  the  power  is  flowing  into  or  out  of 
the  bus.  It  is  the  reversal  of  current  with  respect  to  the  bus 
voltage  that  determines  which  way  to  trip. 

As  before,  Fig.  163  shows  only  the  protection  of  one  wire, 
but  it  must  be  remembered  that  the  others  are  similarly  pro- 
tected with  the  voltage  connected  to  the  bus  which  gives  a 
normal  lag  of  voltage  30  deg.  behind  the  current  at  100  per 
cent  power  factor.  The  necessity  of  this,  as  well  as  the  method 
of  determining  the  correctness  of  this  phase  relation,  has  been 
discussed  previously. 

DISADVANTAGES  OF  CROSS-CONNECTED  SYSTEMS 

The  use  of  cross-connected  relays  or  double-contact  reverse- 
power  relays  has  the  same  disadvantage  as  any  other  balanced 
scheme  in  that  trouble  that  occurs  on  the  busbars  or  on  all  the 
feeders  simultaneously  cannot  be  automatically  cleared. 

Another  disadvantage  is  encountered  when  an  attempt  is 
made  to  cut  in  a  new  feeder.  Assume  that  the  feeders  are 
heavily  loaded  and  arrangements  are  being  made  to  switch 
in  a  new  one.  If  the  attempt  is  made  to  close  the  switch  on 
the  substation  end  first,  the  new  feeder  will  be  tripped  out, 
whereas,  if  it  is  first  closed  in  from  the  generating  station  end, 
the  feeders  carrying  the  load  will  be  tripped  out.  When  only 
one  line  is  in  service,  the  chance  of  tripping  out  the  new  line 
while  switching  it  in  from  the  substation  end  is  the  same  as  the 
chance  of  tripping  out  the  loaded  line  by  doing  the  switching 
from  the  generator  end,  but  in  either  case  there  is  no  danger  unless 
the  loaded  line  is  carrying  a  current  twice  as  great  as  the  relay 
setting.  As  the  number  of  lines  in  service  is  increased  the 


172 


PROTECTIVE  RELAYS 


possibility  of  tripping  out  the  new  line  at  the  substation  be- 
comes greater,  while  the  possibility  of  tripping  out  the  loaded 


Relays  JnstantOpeninq 
and  77>77e  Limit  Closing 


Line 


Line 


FIG.  164. — Connections  of  G.  E.  overload  relays,  double  contact  power  direc- 
tional and  auxiliary  relays  for  protection  of  parallel  lines. 

lines  at  the  generating  station  becomes  less  as  shown  by  the 
following  table: 


Number  of  lines 
in  service 

1 
2 
3 

4 


Number  of  times  load-per-feeder  must  be 
greater  than  relay  setting  in  order  to 

trip  breaker 
at  substation  at  generating  station 

2 


IX 


PROTECTION  OF  PARALLEL  FEEDERS 


173 


174  PROTECTIVE  RELAYS 

The  same  trouble  is  likely  to  be  encountered  in  case  one 
line  should  be  opened  accidentally.  The  obvious  remedy 
for  this  condition  is  to  give  the  overload  elements  of  the  relays 
so  high  a  setting  that  the  normal-load  current  cannot  possibly 
operate  them. 

By  doing  this,  however,  the  previously  stated  advantage 
of  setting  relays  to  trip  on  a  fault  current  less  than  full  load 
is  lost.  It  then  becomes  necessary  to  choose  which  gives  the 
most  desirable  protection.  The  possibility  of  tripping  out 
the  breaker  when  a  new  feeder  is  cut  in  can  also  be  overcome 
by  mechanically  holding  the  substation  relay  of  that  feeder,  while 
cutting  in  first  at  the  substation  and  then  at  the  generating 
station. 

Another  method  of  protecting  parallel  feeders  by  the  differ- 
ential method  is  shown  in  Fig.  164.  This  method  uses  a 
polyphase  double  contact  power  directional  relay  in  connection 
with  three  overload  induction  type  relays.  There  are  in  addition 
two  auxiliary  relays  in  the  trip  circuit  which,  as  will  be  seen  from  a 
study  of  the  connections,  prevent  the  tripping  of  the  second 
breaker  (after  the  first  has  tripped)  due  to  the  rebound  of  the 
disk  in  the  relay. 

Additional  diagrams  illustrating  the  connections  of  a  power 
directional  relay  and  over-current  relay  are  shown  in  Figs.  165 
and  166. 


CHAPTER  XIII 


PROTECTION  OF  RADIAL,  RING  AND  NETWORK 
SYSTEMS 

One  of  the  most  important  uses  of  the  protective  relay  is  to 
localize  and  isolate  a  defective  feeder  or  piece  of  apparatus 
in  a  radial  system  of  transmission,  whether  the  transmission 
be  confined  to  small  units  in  one  building,  or  a  system  cover- 
ing miles  of  lines.  The  principle  of  application,  and  desired 
result,  is  all  the  same.  A  radial  system  is  one  in  which  there 


GENERATORS 


rffELAYS 


FIG.   167. — One-line  diagram  of  simple  radial  system. 

is  a  main  generator  or  generators,  feeding  a  main  bus  (some- 
times sectionalized),  which  in  turn  feeds  several  smaller  busses; 
these  in  turn  each  feed  several  more  feeders  or  machines,  and 
so  on.  Each  subdivision  is  protected  by  circuit  breaker  and 
overload  relays.  Figure  167  shows  the  diagrammatic  layout 
of  a  radial  system.  In  A.C.  transmission  three-phase  is  gen- 
erally used,  but  instead  of  showing  the  three  wires  in  the  diagram, 
the  three  wires  are  represented  by  a  single  line. 

The  main  generators,  A,  B,  and   C,  supply  the  main  bus 
D   with  energy.     From   this  bus   are  shown  two  three-phase 

175 


176 


PROTECTIVE  RELAYS 


lines  E  and  F,  which  in  turn  supply  the  busses  G  and  //.  In 
general  practice  there  will  be  a  large  number  of  lines  taken  from 
bus  D  but  for  the  sake  of  clarity  only  two  are  shown.  The 
busses  G  and  H  may  be  in  substations  many  miles  away  and 
E  and  F  long-distance,  high-tension  transmission  lines,  or 
G  and  H  may  be  distribution  boxes  in  a  factory  or  power  plant 
and  E  and  F  a  few  feet  of  cable  or  bus. 

Tapped  off  the  busses  G  and  H  are  lines  7,  J,  K  and  L,  which 
supply  busses  M,  N,  0  and  P.  These  busses  have  more  feeders 
tapped  on.  Although  at  each  subdivision  only  two  lines  are 

ABC 


CURRENT 
TRANS^ 

CIRCUIT  \ 
3REAKER\_ 

r 

O 

c 

/ERLOAL 
)      <j 

:± 

1  RELAYS 

?   1 

"'',            ( 

: 

:; 

I      i 

1  STRIP  CO/L 

TOD.C.COA 

'TffOL  CIRCUIT 

THREE  PHASED  OR4- 
W/ff£  CIRCUIT 

Rear  Vi'ew 

FIG.  168. — Complete  diagram  of  each  relay  shown  in  Fig.  167. 

shown,  it  must  be  remembered  that  the  number  of  lines  is 
unlimited  electrically.  For  instance,  bus  G  might  supply  a 
dozen  feeders,  and  bus  H  might  supply  another  20  or  30 
feeders  and  busses. 

Immediately  as  each  line  leaves  the  bus,  it  is  supplied  with 
a  circuit  breaker  and  right  after  the  circuit  breaker  comes  a 
current  transformer  which  actuates  an  overload  relay,  which 
in  turn  is  arranged  to  trip  the  breaker. 

As  stated  before,  the  line  E  represents  three  separate  wires 
in  actual  practice.  The  relay  is  in  reality  three  separate  relays 
operated  from  three  current  transformers,  and  the  circuit 
breaker  is  a  three-pole  breaker.  In.  other  words,  if  Fig.  167 
were  fully  drawn  out,  each  feeder,  relay  and  breaker  would 


RADIAL,  RING  AND  NETWORK  SYSTEMS 


177 


look  like  Fig.  168,  which  plainly  shows  the  three  lines  A,  B 
and  (7,  the  current  transformers,  relays,  circuit  breaker  and  trip 
connection. 

Now,  suppose  a  short-circuit  or  rather  a  heavy  overload 
occurred  on  one  of  the  feeders  going  from  bus  N,  the  overload 
extending  all  the  way  back  to  the  generators  as  shown  in  Fig. 
169  by  the  heavy  black  lines. 

Relays  Z,  T  and  Q  would  be  affected,  and  if  they  were  all  set 
the  same,  all  three  breakers  would  go  out.  This  would  kill 


FIG.  169. — Heavy  line  shows  path  of  overload  with  fault  at  "X." 

everything  connected  to  line  E.  Therefore,  breaker  on  line 
E  must  not  go  out  if  a  short  occurs  on  line  N.  Neither  must 
J  go  out.  Only  the  line  in  which  the  short-circuit  occurred 
must  go  out. 

Therefore,  all  relays  connected  to  lines  leading  from  busses 
N,  M,  0  and  P  must  trip  before  relays  connected  in  lines  /, 
J  and  L.  Then  relays  in  these  lines  must  trip  before  relays 
in  lines  E  and  F. 

The  problem,  then,  is  to  select  a  relay  which  may  be  set  to 
give  this  selective-time  action.  Let  us  first  consider  the 
curves  of  an  inverse-time-bellows  relay,  as  shown  in  Fig.  170. 
It  is  very  evident  that  with  various  settings  the  time  will  vary. 

12 


178 


PROTECTIVE  RELAYS 


Say  that  relay  Q  is  set  according  to  curve  3,  relay  T  curve  2, 
and  relay  Z  curve  1.  Now  suppose  the  overload  amounted 
to  200  per  cent  of  full  load.  Relay  Z  trips  first  (in  i  sec.); 
relays  T  and  Q  would  require  4  and  6  sec.  respectively.  So  the 
action  is  perfectly  selective  at  200  per  cent  load. 

Suppose  the  overload  was  400  per  cent.  Relay  Z  still  trips 
first  (in  J^  sec.).  But  an  actual  short-circuit  might  draw 
about  2,000  per  cent  or  in  any. case  over  1,100  per  cent  load. 
Consider  the  curves  at  this  point.  They  intersect.  Therefore, 


200     400    600     800    1000    1200    1400    1600    1800  2000 
Percent  Load  Required  to  Trip 

FIG.  170. — Time  load  curves  of  a  Westinghouse  bellows  type  overload  relay. 

the  action  is  no  longer  selective,  but  at  heavy  overloads  the  action 
is  almost  instantaneous.  Therefore,  relays  having  curves 
similar  to  Fig.  170  are  not  suitable  for  radial  protection. 

Definite  Time. — Let  us  consider  the  definite-time-limit 
relay.  Relay  Z  may  be  set  for  1  sec.,  relay  T  for  2  sec.,  and 
relay  Q  for  3  sec.  It  is  now  evident  that  no  matter  what  the 
overload,  if  the  relays  act  according  to  their  setting,  the  most 
distant  feeder  will  go  out  first.  So  it  would  seem  that  a  definite- 
time-limit  relay  should  be  satisfactory. 

But  it  is  not  necessary  to  trip  a  150  per  cent  load  as  quickly 
as  a  short-circuit.  Nor  is  it  well  to  sustain  a  short  on  line 


RADIAL,  RING  AND  NETWORK  SYSTEMS 


179 


E  for  3  sec.  The  obvious  remedy  is  to  set  the  time  closer.  How- 
ever, there  may  be  four  or  five  subdivisions  of  the  radial  system 
and  practice  has  shown  that  it  is  useless  to  depend  on  a  bellows 
relay  for  such  close  time.  In  the  first  place,  it  takes  considerable 
time  to  test  and  set  the  correct  time,  and  then  it  is  not  per- 
manent as  the  leather  often  dries  out  and  changes  the  time 
so  that  at  a  critical  moment  the  relay  will  not  isolate  its  feeder 
line  properly. 

Inverse-definite-minimum  Time. — Evidently,  the  lines  must 
be  protected  by  an  easily-set,  permanently-accurate  relay  and  one 
which  combines  the  features  of  an  inverse-time-limit  relay  at 


200        400       600        800       1000      \ZQQ      1400 
Percent  Current  Required  to  Close  Contacts 


1600 


FIG.  171. — Time  load  curves  of  a  Westinghouse  induction  type  overload  relay. 

moderate  overloads  with  those  of  a  definite-time-limit  at  heavy 
overloads.  The  induction  relays  described  in  a  previous  chapter 
meet  these  conditions  very  satisfactorily.  Consider  the  curves 
of  the  induction  relay  shown  in  Fig.  171. 

Say  relay  Z  is  set  according  to  curve  3;  relay  T  to  curve  8, 
and  relay  Q  to  curve  10.  Consider  150  per  cent  load.  Relay 
Z  trips  in  1.3  sec.  The  time  is  now  selective.  The  farthest 
relay  takes  0.5  sec.;  the  next  1>£  sec.;  the  next  2  sec.  Thus 
the  time  is  always  selective  with  a  relay  of  this  type.  Figure  172 
gives  curves  of  another  type  of  induction  relay. 

Minimum  Time. — The  next  question  is,  how  many  lines 
or  subdivisions  can  be  protected,  or  in  other  words,  how  close 
can  the  settings  be  and  still  have  accurate  selective  action. 


180 


PROTECTIVE  RELAYS 


It  is  not  safe  to  sustain  a  short-circuit  more  than  2  sec.  on  the 
generator;  this  limits  the  first  division  to  2  sec.  The  last 
subdivisions  may  be  instantaneous.  The  number  of  divisions 


Times  Starting  Current 

FIG.  172. — Time  load  curves  of  a  G.  E.  induction  type  relay. 

in  between  depends  on  the  accuracy  of  the  relay  and  the  time 
it  takes  the  breaker  to  open. 

Figure  173  gives  the  time  taken  by  various  breakers  between 
the  instant  of  tripping  and  opening  of  the  circuit  which  relieves 
the  overload.  It  will  be  noted  that  0.3  sec.  is  a  fair  value  to  allow. 


^r 


0      I      1      3      4      5 


9      10     II      IZ     13     14 


Inches  of  Travel 

FIG.  173. — Typical  time  characteristics  of  oil  circuit  breakers. 
(1)  15,000  volts,  2000  amperes.     (2)  88,000  volts,  300  amperes.     (3)  66,000  volts, 

300  amperes. 

Therefore,  if  the  relays  were  perfect,  each  subdivision  could 
be  set  with  0.3  sec.  to  maintain  selective  action.  But  the 
time  of  the  circuit  breaker  may  vary  slightly,  so  it  is  better 
to  allow  0.4  sec.  Then  allow  0.1  sec.  for  any  inaccuracies 


RADIAL,  RING  AND  NETWORK  SYSTEMS 


181 


which  may  occur  in  the  relay.  This  makes  0.5  sec.  about  the 
lowest  difference  which  can  be  depended  on. 

If  the  main  relays  Q  and  R  are  set  for  2  sec.,  then  S,  T  and  U 
and  V  may  be  set  for  1J^  sec.;  W  to  Z  and  A'  to  Dr  for  1  sec. 
The  next  division  would  be  set  for  J£  sec.  and  the  next  division 
for  instantaneous  trip. 

In  this  manner  a  radial  system  may  divide  four  times  and 
each  division  will  be  thoroughy  protected  and  will  not  cause 
interruptions  to  any  good  feeder  between  the  bus  supplying 
the  defective  feeder  and  the  main  generators. 

Parallel  Feeders  on  Radial  System. — It  is  very  evident, 
with  the  simple  scheme  just  described,  that  if  an  accident 


••/.5SEC 
FIG.   174. — Radial  system  of  parallel  feeders.      (One-line  diagram.) 

occurred  on  feeder  E,  which  caused  an  interruption,  then  every 
further  division  would  also  be  interrupted.  To  avoid  this,  instead 
of  tying  bus  G  to  bus  D  (Fig.  167)  with  a  single  line,  there  would 
be  two  or  more  parallel  tie  lines,  protected  as  described  under 
"Protection  of  Parallel  Feeders."  The  same  protection  by 
parallel  feeders  also  applies  to  all  other  feeders  such  as  F,  I, 
J,  K,  L,  etc. 

In  one  system,  overload  relays  are  installed  every  time  a 
line  leaves  a  station  and  reverse-power  relays  installed  on 
every  line  that  enters  a  station.  The  overload  relays  are  set 
for  decreasing  time  as,  for  instance,  those  leaving  bus  D  for 
2  sec.;  those  leaving  bus  G  for  1.5  sec.;  and  bus  M  for  1  sec.; 
and  considering  a  more  complete  layout  as  shown  in  Fig.  174, 


182 


PROTECTIVE  RELAYS 


set  E'  and  F'  for  0 . 5  sec.  and  G'  and  //'  instantaneous.  The  reverse- 
power  relays  are  set  very  close,  for  instance,  about  0.1  sec.  Now 
if  a  short-circuit  occurs  on  any  line,  the  reverse-power  relays  kick 
out  the  breaker  at  the  receiving  end  in  0.1  sec.  If  the  short  had 
occurred  on  a  feeder  between  G  and  M,  the  overload  relays  at 
G  would  finish  isolating  the  shorted  feeder  in  1.5  sec.,  leaving  the 
good  feeders  to  handle  the  load  without  a  second's  interruption. 
If  the  short  had  been  between  D  and  G,  the  reverse-power  relays 
would  have  isolated  one  end  of  the  defective  line  in  0.1  sec.  and 
the  relays  at  D  would  have  isolated  the  other  end  in  2  sec.,  again 
allowing  the  good  feeders  to  assume  the  total  uninterrupted  load. 


*— -  Indicates  Overload  Relay 

— -  Indicates  Unidirectional  Reverse-Power  Relax 

X     Indicates  Differentially  Connected  Reverse-tower  Relay 

FIG.   175. — Radial  system  forming  network. 


If,  however,  all  the  lines  between  D  and  G  should  become 
shorted,  as  might  be  the  case  if  all  were  carried  on  the  same 
pole  line,  then  the  stations  G,  If  and  N  and  all  their  loads  would  be 
dead,  unless  other  means  were  taken  to  supply  them.  Suppose 
station  G,  instead  of  being  diametrically  opposite  to  station 
H,  should  be  about  an  angle  of,  say,  30  or  40  deg.  It  would  not 
cost  much  to  have  a  line  run  from  G  to  H.  Then  if  all  the  lines 
from  D  to  G  go  out,  the  stations  G,  M  and  N  would  be  fed  over 
the  tie  line  between  H  and  G.  In  a  similar  manner,  if 
the  lines  between  D  and  H  went  out,  then  H's  load  is  assumed 
by  G. 

In  a  similar  manner,  a  line  may  run  from  M  to  0,  so  even  if  all 
the  lines  from  D  to  G,  or  from  G  to  M ,  or  if  G  itself  goes  out  of 
commission,  the  rest  of  the  system  is  still  supplied. 


RADIAL,  RING  AND  NETWORK  SYSTEMS  183 

It  will  be  noted  that  power  may  flow  from  G  to  H  and  from 
H  to  G't  or  from  M  to  0  and  from  0  to  M.  To  protect  these 
feeders  properly  as  well  as  to  protect  any  parallel  feeders  in 
which  the  power  may  flow  in  either  direction,  the  power  differen- 
tial relay  is  used.  This  does  not  discriminate  between  direction 
of  power  flow  as  long  as  the  load  divides  equally  between  the 
two  lines.  But  if  the  power  in  one  line  reverses  due  to  a  short  in 
that  line,  the  relay  immediately  detects  it  and  trips  that  breaker. 
Another  typical  network  system  with  its  time  setting  is  shown  in 
Fig.  175. 

The  Ring  System. — When  several  substations  are  fed  from 
one  main  generating  station,  and  their  geographical  location  is 
favorable,  the  ring  system  forms  one  of  the  best  ways  of  securing 
uninterrupted  service  with  a  minimum  of  expense  for  feeders,  etc. 
The  ring  system  in  its  elementary  form  has  been  treated  under 
the  chapter  on  "Applications  of  D.C.  Power-Directional  Relays," 
while  the  conditions  of  service  experienced  such  as  low  voltage 
and  phase  distortion  have  been  treated  under  the  chapter  on 
"  Characteristics  of  Electrical  Disturbances." 

While  theoretically  it  is  possible  to  include  any  number  of 
substations  in  the  ring,  yet  in  actual  practice  the  number  is 
limited  to  four  or  five  for  two  reasons :  First,  the  best  relays  made 
will  not  select  with  absolute  precision  closer  than  %  °r  J-6  of  a 
second,  and  second,  the  maximum  time  that  a  " short"  should  be 
held  on  before  clearing  is  about  2  sec.  Of  course,  this  rule 
is  not  rigid,  but  represents  the  best  practice. 

Let  us  consider  the  elementary  ring  again  as  shown  in  Fig. 
176.  There  is  a  main  generating  station  at  A  with  substations 
at  B,  C,  D  and  E.  Although  the  transmission  line  is  shown 
by  a  single  line,  for  the  sake  of  simplicity,  yet  it  will  be  under- 
stood that  each  line  if  drawn  out  with  transformers  and  relays 
would  assume  the  appearance  of  the  insert.  At  station  A, 
the  lines  are  protected  by  simple,  accurate  overload  relays, 
while  each  substation  has  A.C.  power-directional  (reverse- 
power)  relays  at  both  ends.  These  relays  are  set  so  that  they 
will  never  trip  when  power  flows  into  the  substation  no  matter 
what  its  magnitude,  but  will  only  trip  when  an  overload  flows 
away  from  a  substation. 

Going  around  the  ring  in  the  direction  A,#,C,D,#,  the  relays 


184 


PROTECTIVE  RELAYS 


on  the  furthest  side  of  each  station  are  set  with  decreasing  time 
element;  for  instance  A  =  2  sec.,  B  =  1J^  sec.,  C  =  1  sec., 
D  =  %  sec.  and  E  =  instantaneous.  Going  around  in  the  direc- 
tion A,#,D,(7,5,  the  relays  on  the  outgoing  sides  would  be  set  as 
follows:  A  =  2  sec.,  E  =  IJ^j  sec.,  D  =  1  sec.,  C  =  %  sec., 
B  =  instantaneous. 

LOAD 


D.C.CON7ROL  CIRCUIT 

Complete  Diagram  of 
Conned-jons  of  Each  Relay 


' RELAYS 


FIQ.  176. — One-line  connection  diagram  of  the  ring  system.     Insert  shows  the 
complete  diagram  of  connections  at  each  station. 

Now  remembering  that  a  relay  starts  to  function  only  when  the 
power  flows  away  from  a  substation,  consider  the  effect  of  a 
"short"  at  X.  Power  starts  to  flow  from  A  through  B  and  C 
into  X  and  also  through  E  and  D  into  X.  Thus  the  relays 
on  the  far  sides  of  B  and  C  start  to  operate,  but  as  C  is  set  for 
1  sec.  and  B  for  1^  sec.  it  is  evident  that  the  breakers  at  C  will 
open  and  relieve  the  "short"  from  both  B  and  C.  On  the  other 
side  of  the  ring,  the  relays  on  the  far  side  of  E  and  D  start  to 
operate,  but  since  D  is  set  for  1  sec.  and  E  for  1J^  sec.  it  is  again 


RADIAL,  RING  AND  NETWORK  SYSTEMS  185 

evident  that  the  breaker  at  D  will  open,  thus  relieving  the  "  short" 
on  both  D  and  E,  and  disconnecting  only  the  faulty  line  between 
C  and  D. 

A  further  study  of  a  " short"  at  any  point  on  the  system  will 
disclose  the  fact  that  this  arrangement  will  localize  and  dis- 
connect only  the  faulty  line  or  substation  and  leave  the  rest 
of  the  ring  intact. 

Parallel  Feeders  on  Ring  System. — When  parallel  feeders 
are  used  between  substations  on  a  ring  system,  the  power- 
directional  relays  not  only  afford  protection  to  the  parallel 
feeders  but  also  serve  to  maintain  the  ring  intact  by  isolating 
the  defective  feeder.  In  ordinary  parallel-feeder  protection 
the  power-directional  relays  were  set  to  operate  almost  in- 
stantly in  order  to  cut  out  the  defective  feeder  as  soon  as  possible, 
but  when  applied  to  the  ring  system,  the  relays  must  be  set 
with  a  decreasing  time  element  as  previously  described. 

Rings  With  More  Than  One  Source. — If,  in  a  ring  system, 
one  of  the  substations  should  be  capable  of  generating  at  cer- 
tain times  as,  for  instance,  might  be  the  case  where  a  substation 
was  provided  with  a  water  turbine  to  take  advantage  of  water- 
power  when  available,  then  the  relay  setting  becomes  a  matter 
of  considerable  study  of  load  division.  If,  however,  this  sub- 
station assumes  the  total  load  of  the  ring,  then  it  is  only  neces- 
sary to  set  the  relays  with  decreasing  time  element  from  this 
station.  As  this  may  be  done  at  telephoned  instructions  from 
the  load  dispatcher,  it  emphasizes  the  necessity  of  a  relay 
which  can  be  quickly  and  accurately  set  without  elaborate 
tests. 

PROTECTION  OF  NETWORK  SYSTEMS 

The  subject  of  the  protection  is  so  broad  and  so  varied  that 
but  little  of  a  general  nature  can  be  said  here.  Each  network 
system  offers  a  deep,  analytical  study  by  a  competent  engineer, 
who  can  take  into  consideration  and  correctly  weigh  all  the  vari- 
ables such  as  division  of  loads  and  overloads,  short-circuit 
currents,  phase  distortions,  and  other  accompanying  effects. 
Even  then,  there  may  be  conditions  which  cannot  be  accu- 
rately predetermined  except  by  the  actual  construction  of  a 
miniature  system  having  the  electrical  characteristics  of  resis- 


186  PROTECTIVE  RELAYS 

tance,  inductance  and  capacity  of  the  original  lines  and  deter- 
mining the  divisions  of  load  from  this  miniature  system. 

In  actual  operation,  it  will  usually  be  found  that  there  are 
several  feeders  or  substations  which  may  be  isolated  in  an  effort 
to  locate  the  trouble,  if  it  is  not  isolated  by  automatic  protec- 
tion. This  condition  may  be  obtained  by  having  a  number 
of  breakers  open  at  the  first  instant  of  trouble.  For  instance, 
in  Fig.  175  is  shown  a  feeder  between  stations  N  and  S  which 
is  used  most  for  maintaining  correct-voltage  regulation.  In 
case  of  trouble,  however,  it  would  be  possible  to  dispense  with 
this  until  the  trouble  was  cleared  up  on  the  rest  of  the  system. 
We  have,  therefore,  assumed  that  the  circuit  breaker  on  section 
A  in  the  substation  is  equipped  with  an  instantaneous  relay.  If 
it  should  happen  that  the  trouble  is  on  this  section  of  line  the 
relay  in  station  S  will  operate  after  ^  sec-  and  clear  the  trouble ; 
but  if  the  trouble  is  not  on  this  particular  feeder,  no  harm 
will  be  done  and  the  load  that  is  supplied  from  it  will  not  be 
interrupted.  In  order  that  synchronizing  and  other  switching 
on  the  system  shall  not  cause  interruptions,  it  is  assumed  that 
the  minimum  time  limit  of  y±  sec.  is  necessary.  If  such  a 
setting  is  used,  and  a  short-circuit  occurs  at  the  point  Z,  the 
relay  in  substation  N  will  require  J^  sec.  to  operate,  and  there 
will  be  a  further  J^  sec-  required  for  the  circuit  breaker  to  open. 
The  relays  at  substation  P  will  not  begin  to  operate  until  the 
switch  at  substation  Nhas  opened,  because  it  is  assumed  that  the 
short-circuit  is  close  to  the  latter  substation  and  there  is,  con- 
sequently, no  unbalancing  at  substation  P.  There -will,  therefore, 
be  still  further  delay  of  J-£  sec.  at  substation  P  before  the  trouble 
is  finally  cleared.  It  is  for  this  reason  that  the  definite  time 
limits  in  the  tie  feeders  between  substations  P,  S  and  T  have 
been  shown  to  be  higher  than  appears  necessary  at  first  sight. 
With  the  setting  shown  in  these  substations  it  will  require  more 
than  2  sec.  to  clear  a  case  of  trouble  should  it  occur  in  either 
section  B  or  C.  For  this  reason  it  may  be  thought  advisable 
to  adjust  the  relays  at  substation  T  so  that  they  have  a  lower 
time  setting,  with  the  result  that  one  of  them  will  operate  on 
practically  all  cases  of  trouble,  but,  as  in  the  case  of  section  A, 
this  will  not  result  in  any  interruption  of  service;  it  will  merely 
trip  out  a  circuit  breaker  that  can  later  be  closed  by  the  attendant. 


RADIAL,  RING  AND  NETWORK  SYSTEMS 


187 


These  illustrations  show  how  to  adapt  relays  to  complicated 
systems,  thus  securing  all  the  advantages  which  can  be  obtained 
from  a  close  interconnection  of  stations  and  substations. 

PROTECTION  OF  SYSTEMS  BY  UNDER-VOLTAGE  AND  EXCESS- 
CURRENT  RELAYS 

Another  system  successfully  employed  to  protect  a  ring 
system  utilizes  a  combination  of  under-voltage  and  excess- 
current  relays.  In  this  system  all  the  breakers  at  each  sub- 
station are  mechanically  locked  in  closed  position,  and  this 
lock  can  never  be  opened  by  excess  current  but  only  by  low 
voltage.  When  the  voltage  falls  to  a  certain  predetermined 
value,  generally  about  70  per  cent  of  normalj  the  under-voltage 


-AUX.SW.OPENMEN 


MAGNETICALLY-- 

V  €J|    ,/  1  BREAKER  IS  OPEN 

OPERATE. 

7  LOCK 

\      \  //          + 

1  \mfH-sfJ  OVERLOAD  TRIP 

POTENTIAL  U  J  L^* 
TRAHS.      tC*VT  r*"" 

STATION  SERVICE 
TRANS. 

]^l    ft        ATTACHMENT 
1    ^^®r«^  TO  CURRENT 
|       J*~*  §  TRANSFORMERS 

4-POLE  SWITCH  r 
TOCUTOUTALLl 
LOCHS  A  RELAYS 

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LOCKS 

1  CIRCUIT  BRIAKt  R  \  >•  jo  (jrHFffinriff 
^l   TRIP  ARM        I     £>TOOTHtK  LOCKS 

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—  u  — 

UNDERVOL  TAGE  RELA  Y 

FIG.  177. — Connection  diagram  for  protection  by  over-current  and  under-voltage 

relays. 

relay  operates  and  opens  the  lock  on  the  breaker;  then  an  excess 
current  operating  the  excess-current  relay  is  free  to  trip  the 
breaker.  The  object  of  this  arrangement  is  that  the  voltage 
will  fall  only  in  the  immediate  vicinity  of  the  short-circuit, 
so  although  the  short-circuit  current  extends  all  the  way  back 
to  the  generators,  yet  only  the  breakers  in  the  immediate  vi- 
cinity of  the  short-circuit  will  be  unlocked  and  allowed  to  trip 
upon  the  excess  current. 

When  stations  are  quite  close  together,  the  breakers  in  an 
unaffected  section  may  sometimes  be  unlocked  on  the  occurrence 


188  PROTECTIVE  RELAYS 

of  a  heavy  disturbance,  but  in  this  case,  the  short-circuit  will 
usually  be  localized  by  the  inverse-time-element  selectivity 
of  the  excess-current  relays,  which  will  clear  the  trouble  before 
the  other  relays  trip  the  unlocked  breakers. 

Figure  177  shows  the  diagram  of  connections  of  this  com- 
bination. The  potential  transformers  supply  the  under- 
voltage  relays  which  normally  have  their  contacts  open. 
The  auxiliary  relay  is  energized  by  a  service  transformer 
and  this  relay  in  turn  operates  the  electrical  locks  on  the  breaker 
using  the  same  transformer  for  its  energy  supply.  Both  the 
auxiliary  relay  and  the  locks  on  the  relays  are  normally  energized 
and  are  de-energized  either  by  the  functioning  of  the  under- 
voltage  relays  or  the  loss  of  station  voltage.  As  is  shown 
in  the  diagram  one  set  of  relays  may  control  a  number  of  circuit- 
breaker  locks,  and  each  lock  is  provided  with  an  individual 
cut-out  switch  and  a  pallet  switch  to  open  its  circuit  after 
functioning. 

When  the  line  potential  drops  to  a  predetermined  value, 
one  or  more  of  the  under-voltage  relays  close  their  contacts, 
thus  de-energizing  the  auxiliary  relay,  which  in  turn  de-energizes 
the  electrical  locks  on  the  breaker,  thus  releasing  the  latch 
and  leaving  the  breaker  free  to  be  tripped  by  the  excess-current 
relays.  If  the  voltage  does  not  fall  low  enough,  then  the 
breakers  cannot  be  opened  by  an  excess  current. 


CHAPTER  XIV 
MISCELLANEOUS  RELAYS 

Over-  and  Under-voltage  Relays. — Oftentimes  there  are 
conditions  of  abnormally  high  or  low  voltage  which  may  lead 
to  disastrous  results  if  not  promptly  detected  and  corrected. 
Considerable  damage  might  easily  be  done  by  an  increase  in 


FIG.  178. — Induction   type,    over-    and   under-voltage  relay.     (Westinghouse.) 

voltage  burning  out  lamps.  This  increase  may  be  due  to  a 
number  of  causes,  such  as  the  failure  of  a  Tirrill  regulator 
on  the  generator,  or  a  short-circuit  in  the  generator  field  resistor, 
etc.  Then,  on  the  other  hand,  the  voltage  may  drop  low, 
and  even  if  this  drop  did  not  lead  to  disastrous  overload  results, 
it  might  cause  considerable  annoyance  due  to  lamps  burning 
dim  or  motors  running  slow. 

189 


190 


PROTECTIVE  RELAYS 


To  give  warning  of  such  abnormal  conditions,  or  actually 
to  disconnect  a  circuit,  should  practice  so  determine,  an  over- 
voltage  or  an  under-voltage  relay  may  be  installed.  Such  a 
relay  is  shown  in  Fig.  178.  It  is  the  same  in  construction  and 
principle  of  operation  as  the  previously  described  overload 
relays,  except  that  the  windings  are  wound  to  stand  the  im- 
pressed voltage.  In  the  over-voltage  relays,  the  contacts 

close  when  the  voltage  exceeds  a 
certain  predetermined  point,  which 
either  trips  a  breaker,  rings  a  gong, 
or  gives  some  other  signal  to  the 
operator  that  the  voltage  is  too  high 
and  requires  attention. 

The  tripping  voltage  may  be  varied 
over  wide  ranges  generally  between 
75  per  cent  and  160  per  cent  of 
normal,  and  a  time  delay  may  be 
obtained  if  desired.  So  if  a  circuit 
is  normally  running  at  110  v.,  it  be- 
comes possible  to  trip  it,  or  ring  a 
bell,  if  it  goes  to  115  or  120  v.  or 
higher. 

In  the  under-voltage  relay,  the 
windings  are  arranged  so  that  the 
voltage  tends  to  keep  the  contacts 
open.  Then  should  the  voltage  drop 


FIO.  179. — Solenoid  type  un-   to  75  or  80,  or  whatever  the  relay 

BESS"*    relay'         ^^    ig    S6t>     thefe    i8     n°     lonSer     t0r<!Ue 

enough  to  hold  them  open,  so  they 
close  and  generally  sound  an  alarm. 

The  Solenoid  Relay. — The  solenoid  principle  as  well  as  the 
induction  principle  may  be  used  to  indicate  the  conditions  of 
over-voltage  and  under-voltage,  but  its  action  is  much  rougher 
in  adjustment  than  the  induction.  In  the  over-voltage  relay 
the  plunger  is  arranged  to  move  upward  and  close  contacts  on 
over-voltage.  On  the  under-voltage,  the  solenoid  normally  holds 
the  plunger  up  and  contacts  open,  but  on  a  fall  in  voltage,  the 
plunger  drops  and  the  contacts  close.  A  solenoid  relay  is  shown 
in  Fig.  179. 


MISCELLANEOUS  RELAYS 


191 


In    connecting,    the   potential    circuit   is    connected    directly 

across  the  line,  like  a  voltmeter;  or  in  the  case  of  high-tension 

work  it  is  connected  to  the  secondary 

of  the  potential  transformer. 

Under-current  Relays. — In   such  cir- 
cuits as  the  constant-current  arc  system, 

some  signal  must  be  given  if  the  current 

drops   too  low.      In  other   places  there 

may  be  conditions  which  require  an  in- 
dication   of    low   current.      To   provide 

this,   a  regular  induction-type   relay  is 

used,  except  that  the  current  tends  to 

keep  the  contacts  open.      Then  if  the 

current  drops  to  a  certain  predetermined 

value  there  will  no  longer    be   enough 

current  to  hold  them  open,  and  thus  they 

close  and  give  a  signal  or  trip  a  breaker. 

These  relays  are  also  used  to  shut  down 

automatic  substations. 

Overload   Telegraph  Relay.— In  Fig. 

180  is  shown  a  type  of  simple  overload 

relay  which  is  connected  by  circuit  to  a  shunt.  In  the  diagram- 
matic scheme  of  parts,  Fig.  181,  the 
iron  armature  A,  carrying  the  contact 
B  and  pivoted  at  C,  is  held  in  its 
normal  position  (contacts  B  and  D 
open)  by  the  tension  of  the  spring 
E.  This  spring  is  attached  to  an 
adjustable  arm  F  secured  to  the  frame 
G  by  the  thumbscrew  H.  Arm  F 
carries  a  scale  calibrated  in  millivolts. 
The  arm  that  carries  contact  D  is 
insulated  at  I  from  the  main  frame 
G.  The  terminals  K  and  K1  of  the 
coil  J  are  connected  to  a  shunt  which 
is  in  series  with  the  line,  and  there- 
fore takes  a  current  proportional  to 

the  main  current.     If  the  relay  is  set  for  50  m.v.,  then,  when 

the    current    in    the    shunt   produces   a  drop  of  50  m.v.  the 


FIG.  180. — Westinghouse 
'telegraph"  type  D.C.  over- 
load relay.  Instantaneous 
trip. 


K, 


FIG.  181.— Schematic  dia- 
gram of  overload  "telegraph" 
type  relay. 


192 


PROTECTIVE  RELAYS 


armature  A  is  attracted,  closing  the  contacts  B  and  I),  which 
closes  a  circuit  from  L  and  L,  to  the  circuit  breaker's  trip  coil. 
Figure  182  gives  the  diagram  of  connections  of  the  relay  and 
circuit  breaker  under  normal  load,  the  path  of  the  currents 
being  shown  by  arrowheads.  An  overload  causes  the  relay 
contacts  to  close  the  trip-coil  circuit  to  the  breaker  and  the 
latter  opens  the  circuit,  thus  relieving  the  overload  on  the 
system.  In  installations  where  the  potential  trip-coil  circuit 
is  connected  to  the  circuit  to  be  controlled,  the  overload  trip 


FIG.  182. — "Telegraph"  type  relay  connected  in  the  circuit. 

attachment  on  the  breaker  should  always  be  connected  in  the 
circuit,  since  dead  short-circuit  on  the  line  may  cause  the  volt- 
age to  drop  so  low  that  it  will  not  operate  the  potential  trip 
coil  on  the  breaker.  This  allows  the  overload  attachment 
on  the  breaker  to  be  set  high,  for  protection  against  short- 
circuits  or  other  violent  disturbances,  but  the  relay  is  set  so  as 
to  give  protection  against  moderate  overloads. 


PROTECTION  AGAINST  REVERSED  PHASE 

If  a  three-phase  motor  is  disconnected  from  a  circuit  and  the 
phases  reversed  when  it  is  reconnected  it  will,  naturally,  run  back- 
ward. Such  a  reversal  may  occur,  and  has  occurred,  when 
the  motor  is  disconnected  for  repairs,  through  an  error  in  recon- 
necting loads  at  the  power  house,  or  substation,  or  from  a  number 
of  other  causes. 


MISCELLANEOUS  RELAYS 


193 


In  many  cases  the  reversal  of  rotation  of  a  motor,  aside  from 
the  inconvenience  it  causes,  is  not  a  serious  matter  as  the  error 
can  be  corrected  at  the  motor  terminals.  In  other  cases,  however, 
serious  consequences  may  result.  The  reversal  of  an  elevator 
motor,  for  instance,  might  result  in  wrecking  the  machinery 
and  loss  of  life. 

To  protect  motors  against  phase  reversal  where  such  protec- 
tion is  necessary,  the  reverse-phase  relay  has  been  developed. 


, 


FIG.  183. — Reverse-phase  relay.     (Westinghouse.) 

This  is  shown  in  Fig.  183.  The  parts  are  all  identical  with  the 
overload-induction  relay  with  the  exception  of  the  windings. 
The  main  coil  is  a  voltage  coil  exactly  like  a  watt-hour  meter,, 
and  the  series  coil  is  of  heavy  wire,  connected  to  the  secondary 
of  a  small  step-down  transformer.  Since  one  coil  is  tapped  on 
one  phase  and  one  on  the  other,  they  have  the  necessary  phase 
displacement  to  produce  a  shifting  field,  which  reacts  on  the 
disk.  The  external  connections  are  shown  in  Fig.  184.  Nor- 

13 


194 


PROTECTIVE  RELAYS 


mally,  the  voltage  tends  to  rotate  the  disk  to  the  right  and  keep 
the  contacts  open.  But  should  one  phase  be  reversed,  or  should 
one  phase  fail,  or  should  the  voltage  drop  below  75  per  cent 
of  normal,  then  the  contacts  close  and  open  a  breaker.  Reversal 
of  a  phase  actually  reverses  the  direction  of  rotation,  causing 
the  contacts  to  close  very  quickly.  This  relay  will  not  prevent 
a  three-phase  motor  from  running  single-phase  if  one  phase 
opens  while  running.  It  will,  however,  prevent  the  motor 


Li 


ne 


Vottvrge  Transformers 
when  used 

FIG.   184. — Schematic  diagram  of  connection  for  reverse-phase  relay. 

from  starting  again.  In  the  case  of  an  elevator  motor  this  is  an 
advantage  as  it  allows  the  car  to  run  to  a  floor  and  stop  even 
though  one  phase  should  open  while  the  car  is  between  floors. 

Service -restoring  Relays. — There  are  many  cases  in  which 
it  is  necessary  actually  to  open  the  breaker  to  clear  a  short- 
circuit  as,  for  instance,  an  arc  across  two  lines,  which  is 
killed  the  instant  the  breaker  opens.  This  permits  the  feeders 
to  be  put  back  in  service  immediately. 

If  the  circuit  breaker  is  reclosed  automatically  within  a 
second  after  the  transient  trouble  has  occurred,  the  service 
will  be  restored  in  time  to  prevent  induction  motors  from 
stalling. 


MISCELLANEOUS  RELAYS 


195 


The  service-restoring  relay  system  has  been  developed  to 
perform  this  operation  within  the  shortest  possible  time  and 
thus  reduce  all  disturbances  to  a  minimum,  thereby  greatly 
improving  the  service.  Should  a  permanent  defect  occur, 
the  system  will  allow  the  breaker  to  remain  open  until  the 
defect  is  cleared. 

A  schematic  diagram  of  operation  is  shown  in  Fig.  185. 
Any  type  of  overload  relay  may  be  employed  to  trip  the  circuit 


Circurf  Breaker 


Feeder 


Closing  Co  if  -?fl00l'A 

Transwrme  <" 


D.  C,  forrirolCircuii- 


Restoring  Re /ay 
Resistor 


V/NAA 


-Voffaae  Transformer 


FIG.  185. — Connections  of  automatic  service-restoring  system. 

breaker  on  overload  as  previously  described.  A  voltage  trans- 
former on  the  feeder  outside  the  circuit  breaker  is  connected  so 
that  its  potential  opposes  that  of  another  voltage  transformer 
connected  to  the  busbars.  The  restoring  relay,  which  is 
similar  to  a  magnet  switch,  is  connected  in  series  with  these 
two  voltage  transformers.  Before  a  short-circuit  occurs,  both 
voltage  transformers  are  subjected  to  the  same  conditions 
so  that  no  current  will  flow  through  the  restoring  relay;  but 
when  a  short-circuit  occurs  and  the  circuit  breaker  has  been 
opened  by  the  overload  relay,  current  will  be  forced  by  the 
busbar  transformer  B  into  the  feeder  transformer  A,  through 
the  restoring  relay.  The  restoring  relay  will  then  close  its 


196 


PROTECTIVE  RELAYS 


contacts,  which,  in  turn,  will  close  the  circuit  breaker,  which, 
of  course,  must  be  of  the  electrically  closing  type  as  well  as 
electrically  tripping. 

In  case  of  a  permanent  defect  on  the  feeder,  the  restoring 
relay  would  continue  to  open  and  close  the  circuit  breaker 
indefinitely,  as  each  time  the  breaker  closes,  the  overload 
relay  opens  it.  To  prevent  this,  a  limiting  relay,  similar  to  the 
over-voltage  relay,  but  equipped  with  weaker  spring  and  heavier 
damping  magnets  so  that  its  action  is  sluggish,  is  connected 


Circuit-Breaker 


Feeder- 


Graphic  Ammeter 
)Kesistors    used  as  Operation 
Umitma'Relay        Border 


Position 


^Voltage  Transformer 

Handle  End 

O 


Transformer 


-Z-Pole 
Knife 
Switch 


<_ 

Dragging  _  13  o 
Contact      \*  o 


J     \     I    \    4  \ 

O  .0       10  1  O         |0  I 


_ 

e»  o   ,0.0    |o,o    ;x 
Jo   'o  '  o    10  1  o    i  ^ 


pOSit/0ns  of 
Control  Switch 


\To  C/ose/rom  tttyoa  4$  To  Trip  \ 
6reaker\  PositioirPosition  Breaker* 


FIG.   186.  —  Complete  diagram  of  connections,  showing  control  switch,  graphic 

ammeter,  etc. 

in  such  a  manner  that  while  the  circuit  breaker  is  open  it  is 
subjected  to  the  same  difference  of  potential  that  is  operating 
the  restoring  relay.  Every  time  the  circuit  breaker  opens,  the 
limiting  relay  contacts  begin  to  close  and,  due  to  its  heavy 
damping,  they  do  not  return  to  the  starting  point  immediately 
after  the  circuit  breaker  is  closed.  After  the  circuit  breaker 
has  opened  and  closed  a  predetermined  number  of  times,  this 
relay  closes  its  contacts,  thus  short-circuiting  the  restoring  relay 
and  preventing  further  operation. 

This    system  is    often    installed    at    substations    having    no 


MISCELLANEOUS  RELAYS 


197 


attendant.  In  that  case  it  is  often  found  advisable  to  have  an 
indicating  device  that  will  show  when  the  service  has  been 
momentarily  interrupted.  For  this  purpose,  a  graphic  ammeter 
is  placed  in  the  direct-current  control  circuit  of  the  circuit 
breaker.  This  will  indicate  whenever  the  breaker  has  been 
closed  by  automatic  means.  Complete  diagrams  are  shown 
in  Fig.  186. 

When  this  system  is  used,  and  provision  is  made  for  tripping 
the  breaker  manually  by  a  control  switch,  contacts  are  arranged 


Zero  Line  of  Current 
in  D.C.  Control 
Circuit.^ 


Power  removed 
from  Trip  CoiL 


"Closing  Coil       Restoring  Relay 
Energized.  Opened. 


FIG.   187.— Oscillograph    record    showing    operation    on    a    transient    "short." 

on  the  control  switch  which  automatically  open  the  circuit 
between  the  two  voltage  transformers,  which  prevents  the 
breaker  from  resetting  immediately  after  it  is  tripped. 

Figure  187  shows  an  interesting  oscillograph  record  of  the 
behavior  of  the  various  parts.  The  time  is  shown  on  the  third 
horizontal  line.  The  second  horizontal  line  shows  the  great 
intensity  of  the  short-circuit  current.  At  the  end  of  0.1  sec. 
the  fourth  line  shows  the  relay  has  acted  and  the  trip  coil  is 
energized.  At  the  end  of  another  0.1  sec.  the  breaker  has 
opened,  thus  decreasing  the  short-circuit  current  to  zero  (second 
line)  in  a  total  of  0.2  sec.  Instantly  the  service  restoring  relay 
is  energized  (first  horizontal  line)  and  in  0.1  sec.  it  closes  its  con- 


198  PROTECTIVE  RELAYS 

tacts,  thus  energizing  the  closing  coil  of  the  breakers.  This  takes 
considerable  current  and  0.4  sec.  as  will  be  seen  from  the  fourth 
line.  Immediately  on  the  closing  of  the  breaker,  the  current 
in  the  restoring  relay  ceases  (first  line);  the  closing  coil  is 
de-energized  and  the  current  continues  normal  in  the  feeder  as 
shown  in  the  second  line. 

Thus  it  will  be  seen  that  only  0.7  sec.  is  required  from  the 
first  instant  of  overload  to  the  instant  of  resumed  service. 

If,  however,  the  short-circuit  is  not  removed  upon  the  opening 
of  the  breaker,  then  when  the  breaker  recloses,  the  short-circuit 


~s*  v  Current  in  uiostna  f  i  >         Lurrgfii  in  (s 

.X^  <to//   J.J-'  I  \  Coil. 

Trip  Coil  Power  removea        Rtstorinq  Relay  Trip  Colt         Power  removea 

intrqiiUL  Irgm  Trip  Coil.  Opened.  Energized.       from  Trip  Coil. 


Energized.       from  Trip  Coil. 

Fio.  188. — Oscillograph   record  showing  operation   on   a   permanent   "short." 

current  immediately  operates  the  overload  relay  and  again  trips 
the  relay  until  the  sluggish  limiting  relay  prevents  further  action. 

This  is  plainly  shown  in  Fig.  188,  where  the  short  is  shown 
interrupted  as  before,  and  the  breaker  closed  again,  but  instead 
of  resuming  normal  current,  the  current  is  again  excessive 
as  shown. 

The  service-restoring  system  has  proved  its  effectiveness 
in  many  cases  and  is  recognized  as  an  invaluable  aid  in  securing 
cheap,  efficient  and  effective  service. 

Bell-ringing  Relays. — If  a  circuit  breaker  is  tripped  out, 
and  the  station  attendant,  instead  of  being  near  at  hand  where 
he  can  see  and  reset  it,  is  some  distance  away,  a  bell-ringing 
relay  may  be  used.  This  must  ring  the  bell  or  alarm  until 
some  notice  or  action  is  taken  if  the  breaker  opens  due  to  the 
protective  relays  tripping,  but  it  is  not  necessary  to  ring  the 
bell  if  the  breaker  has  been  opened  intentionally.  The  relay 
shown  in  Fig.  189,  with  the  cover  removed,  will  meet  the  fore- 
going requirements.  A  schematic  connection  diagram  is  given 
in  Fig.  190  and  the  operation  is  as  follows: 


MISCELLANEOUS  RELAYS 


199 


The  two  electromagnets  A  and  B  are  arranged  to  attract 
the  iron  armature  C  pivoted  at  D,   which  closes  contacts  E 


FIG.  189. — Bell-ringing  relay  for  circuit  breakers. 


CD 


-v-~- 

)           ( 

> 

) 

)                    ( 

) 

)             ( 

> 

)           ( 

> 

)             C 

1        1 

1  J 

i  » 

Ei      iF 

O 

k 

<-:>7b  DireG-fcCurrerrf- 
Trip/ng  Circuit" 

FIG.   190. — Diagram  of  connection  for  bell-ringing  relay. 

and  F  when  attracted.     The  solenoid  A  is  placed  in  series  with 
the  trip  coil  and  relay  contacts  of  the  breaker,  so  that  when  the 


200  PROTECTIVE  RELAYS 

relay  closes  it  energizes  solenoid  A.  This  attracts  armature  C 
and  closes  both  contacts  E  and  F.  Contact  E  closes  the  circuit 
to  solenoid  B  through  a  resistance  G,  and  contact  F  closes  the 
circuit  to  a  bell  or  alarm  H.  Now,  even  though  the  circuit 
to  A  is  opened,  as  it  would  be  if  the  breaker  opened,  solenoid 
B  still  holds  the  contacts  closed  and  the  bell  will  continue  to 
ring  until  the  switch  /  is  opened  for  an  instant,  which  allows  the 
armature  to  drop  and  the  contacts  to  open.  These  relays 
can  be  used  when  a  direct-current  circuit  is  available  for  tripping 
the  breaker.  It  will  also  be  seen  that  should  the  breaker  be 
tripped  by  hand,  the  relay  cannot  operate,  consequently 
the  bell  rings  only  on  automatic  tripping. 

Temperature  Relays. — When  large  power  units  are  used 
in  generating  and  transforming  electric  energy,  some  means 
must  be  employed  to  keep  the  windings  cool  when  they  are 
heavily  loaded,  as  the  capacity  of  a  machine  is  limited  largely 
by  the  maximum  temperature  which  the  insulation  will  stand. 
From  this  it  follows  that  if  apparatus  is  installed,  to  remove 
the  excess  heat  and  keep  the  temperature  within  allowable 
limits,  then  a  given  machine  will  have  a  greater  capacity  than 
one  in  which  no  such  cooling  devices  have  been  employed. 

There  are  several  methods  of  cooling  electric  machines.  The 
simplest  is  by  immersing  the  whole  machine  in  a  tank  of  oil 
(as  in  the  case  of  an  oil-cooled  transformer)  where  the  heated 
oil  rises  to  the  top,  cools  off  and  sinks  again  to  carry  away 
more  heat.  The  cooling  of  the  oil  is  sometimes  hastened 
by  installing  coils  of  pipes  in  the  top  of  the  oil  and  pumping 
cold  water  through  the  pipes. 

It  is  obvious  that  these  methods  cannot  be  used  for  genera- 
tors or  motors,  so  cold  air  is  resorted  to,  large  motor-operated 
blowers  forcing  the  air  through  suitable  ducts  or  channels  in 
the  iron  and  windings. 

In  a  few  instances,  the  light-load  losses  have  been  so  low 
that  there  is  no  necessity  for  operating  the  blower  motors 
when  the  machine  is  unloaded.  Neither  is  it  necessary  to  start 
them  for  a  short-time  heavy  load.  It  is  when  the  units  get 
hot  that  the  blowers  must  start.  For  this  purpose  temperature 
relays  may  be  used.  They  start  the  blower  motors  when  the 
protected  apparatus  reaches  a  certain  high  temperature  and 


MISCELLANEOUS  RELAYS 


201 


continue  blowing  until  the  apparatus  is  cooled  down  to  a 
certain  degree.  In  other  cases,  the  relays  are  used  to  trip  out 
the  breaker,  or  to  ring  a  signal  bell  informing  the  attendant 
that  the  machine  is  too  hot  and  needs  a  revision  of  load  or  another 
unit  cut  in  service.  Sometimes  even  this  cutting  in  of  a  new 
unit  is  done  automatically  when  the  temperature  reaches  a 
certain  limit  and  the  extra  machine  cut  out  when  the  tempera- 
ture goes  down.  The  great  advantage  of  temperature  relays 
over  overload  relays  is  that  a 
motor  or  generator  can  be  held  in 
until  it  gets  hot  enough  to  start 
actual  deterioration  of  the  insula- 
tion. 

D.C.  Temperature  Relays.— To 
accomplish  this  control  without 
the  use  of  thermometers  or  any 
other  time-old  methods  of  measur- 
ing temperature,  advantage  is 
taken  of  the  fact  that  the  resis- 
tance of  copper  wire  varies  with  the 
temperature.  Then  a  Wheat- 
stone's  bridge  arrangement  is  used 
with  a  sensitive  direct-current  re- 
lay instead  of  a  galvanometer;  the 

copper  wire  is  wound  on  a  card  and  placed  in  a  spot  in  the  trans- 
former or  protected  apparatus  where  the  hottest,  or  rather 
indicative,  temperature  is  liable  to  be,  and  when  its  resistance 
reaches  a  certain  amount,  the  relay  closes  its  contacts  by  un- 
balancing the  bridge.  The  copper-wound  card  is  called  the 
exploring  coil. 

The  principle  of  operation  of  the  D'Arsonval  type  is  shown  in 
Fig.  191.  The  main  casting  A-A'  has  an  iron  core  B  producing 
an  annular  gap  C  in  which  the  moving  coil  D  turns  on  jewelled 
bearings.  The  magnet  A-A1  is  magnetized  by  the  coil  E,  which, 
in  practice,  is  connected  directly  across  the  line  or  potential. 
The  moving  coil  D  has  a  contact  F  which  touches  G  when  it 
moves  one  way  and  H  when  it  moves  the  other 

Figure  192  shows  the  diagram  of  connections.  A,  A!  and  A" 
are  resistance  units  wound  with  wire  having  a  zero  temperature 


FIG.  191. — Internal  diagram  of 
D.C.  temperature  relay. 


202 


PROTECTIVE  RELAYS 


coefficient  and  are  each  equal  to  resistance  of  the  copper  wire 
exploring  coil  B  at  its  normal  temperature.  The  four  resistors 
are  connected  in  bridge  arrangements,  the  potential  being  sup- 
plied at  1  and  2  and  the  moving  coil  (in  place  of  galvanometer) 
at  3  and  4. 


COIL 


G    .17777..'. 
->U\UUll 

6 


FIG.   192.  —  External  diagram  of  connections  of  D.C.  temperature  relay. 

For  a  moment,  consider  Figs.  193  a,  b  and  c.  In  the 
first,  Fig.  193a,  the  resistance  of  the  exploring  coil  is  less  than 
the  other  three  so  current  flows  from  left  to  right.  Then, 
as  it  gradually  gets  warmer,  its  resistance  increases  until  it 


X= EQUAL 


X=HIGH 


FIG.   193. — Showing   the    direction   of   current  in   the    "moving-coil"    circuit. 

becomes  equal,  when  no  current  flows  (Fig.  1936).  A  further 
increase  in  resistance  (temperature)  causes  the  current  to  reverse, 
and  the  higher  the  resistance,  the  greater  the  current  in  the 
moving  coil. 

Now  returning  to  Fig.  192,  it  will  be  seen  readily  that  if  the 


MISCELLANEOUS  RELAYS  203 

current  causes  the  coil  to  turn  to  the  right,  then  when  it  reaches 
a  certain  amount,  the  contacts  G  will  close.  This  completes 
the  circuit  to  the  closing  coil  I  which  closes  a  breaker  and  starts 
the  motors  blowing,  or  rings  a  bell,  etc.  When  the  breaker 
closes,  auxiliary  contacts  open  the  circuit  to  relieve  the  relay 
contacts.  The  apparatus  gradually  gets  cooler  and  the  resistance 
of  the  copper  wire  exploring  coil  which  is  wound  in  the  machine 
decreases  until  the  current  in  the  moving  coil  has  reversed 
to  such  an  extent  that  contact  H  is  closed.  This  completes 
the  circuit  to  the  trip  coil  J  and  the  breaker  opens. 

If  the  voltage  of  the  D.C.  circuit  varies,  it  will  affect  the 
accuracy  somewhat,  but  this  error  is  extremely  small  at  the 
balancing  or  reversing  point,  consequently  this  is  the  point 
generally  chosen  at  which  the  relay  closes  its  contacts  on  high 
temperature.  While  this  arrangement  may  be  used  to  protect 
either  A.C.  or  D.C.  apparatus,  it  requires  a  constant  D.C. 
source  for  its  operation. 

A.C.  Temperature  Relays. — When  it  is  desirable  to  have 
the  relay  trip  out  the  circuit  in  the  event  of  excessive  tem- 
perature, the  A.C.  temperature  relay  is  generally  used  as  it 
may  be  arranged  to  trip  out  the  circuit  on  high  temperature, 
but  only  if  the  excessive  current  is  still  flowing.  For  instance, 
say  it  is  a  generator  that  is  being  protected.  The  exploring 
coil  would  be  wound  and  imbedded  in  the  stationary  part  and 
consequently  attain  the  same  temperature  as  the  part  in  which 
it  is  imbedded.  Due  to  a  quick,  heavy  load,  one  portion  of 
the  machine  may  attain  a  quite  high  temperature,  but  before 
this  temperature  can  reach  the  search  coil,  the  load  may  decrease. 
But  the  heat,  in  dissipating  from  the  hottest  part,  may  still 
continue  to  raise  the  temperature  of  the  cooler  parts,  and  then 
in  a  short  time  the  search  coil  may  get  hot  enough  to  trip  the 
relay.  Still,  there  being  very  light  load,  the  machine,  as  a  whole 
is  actually  cooling  and  will  continue  to  cool. 

This  shows  the  necessity  of  using  a  relay  that  will  not  trip 
when  the  temperature  is  high  unless  there  is  also  a  heavy  load; 
nor  will  it  trip  on  a  high  load  unless  the  temperature  is  high. 

It  takes  both  high  temperature  and  high  load  to  operate  the 
relay.  The  connections,  internal  and  external,  of  this  relay 
are  shown  in  Fig.  194.  This  shows  the  regular  induction-type 


204 


PROTECTIVE  RELAYS 


element  A  with  disk  B  arranged  in  the  regular  manner,  so  that 
movement  of  the  disk  closes  the  contacts  C.  It  will  be  noticed 
that  the  current  transformer  supplies  two  circuits;  one  excites 
the  main  coil  of  the  relay  and  the  other  furnishes  potential 
to  the  bridge  arrangement.  This  bridge  arrangement  consists 
of  two  unchanging  arms  D  and  E  (usually  placed  inside  the 


TO  D.C.  JR(P  SOURCE 


LIHE 


LOAD 
FIG.   194. — Connection  diagram  of  A.C.  temperature  relay. 


relay)    and    two    search  coils  imbedded  in  the  winding  of  the 
generator  or  transformer  being  protected. 

The  other  actuating  winding  of  the  relay  (on  the  two  poles) 
is  used  in  place  of  the  galvanometer.  Since  the  torque  on  the 
disk  is  the  reaction  or  the  multiplication  of  the  currents  in  the 
two  windings,  it  is  very  evident  that  even  though  there  is  a 
heavy  current  flowing  in  the  main  coil,  it  will  not  trip  if  there 
is  no  current  in  the  other  two  poles  (due  to  unbalance  of  bridge) ; 
neither  can  the  relay  act  if  the  search  coils  get  hot  enough  to 


MISCELLANEOUS  RELAYS  205 

unbalance  the  bridge  and  force  current  through  the  two  poles 
unless  there  is  a  current  flowing  in  the  main  coil. 

This  shows  that  the  only  condition  that  will  trip  the  relay 
is  excess  temperature  and  heavy  current. 

Temperature  relays  may  seem  like  an  added  expense  and 
luxury  to  the  average  power  plant,  but  a  careful  survey  of 
the  situation  may  reveal  certain  conditions  which,  by  their  elimina- 
tion, will  soon  pay  for  an  installation  of  good  relays.  For 
instance,  note  how  many  hours  the  generator  or  transformer 
is  operating  at  light  load  with  the  blower  motors  working  at 
full  capacity.  Note  how  much  power  they  take  and  the  saving 
that  could  be  made  by  automatically  stopping  the  blowers 
when  they  are  not  required.  The  cost  of  burnt-out  units 
might  also  be  classed  as  a  case  where  temperature  relays  will 
effect  a  great  saving,  as  there  are  many  cases  where  the  relays 
will  give  a  signal  that  the  apparatus  is  approaching  a  danger- 
ous temperature,  thus  enabling  the  attendant  to  act  quickly 
and  distribute  the  load  so  that  the  machine  in  question  is 
relieved.  Of  course  temperature  relays  are  not  an  absolute 
guarantee  against  burnouts,  as  unavoidable  accidents  are  always 
liable  to  occur;  but  if  the  relays  will  prevent  even  one  burnout 
which  might  occur  were  the  relays  not  installed,  then  they  will 
have  paid  for  themselves  both  in  monetary  value  and  satis- 
faction of  operation  of  the  power  plant. 

RELAY    SWITCHES 

In  all  the  relays  so  far,  we  have  assumed  that  the  contacts 
themselves  closed  the  circuit  to  the  trip  coil  of  the  breaker,  but 
when  the  breakers  are  large  and  require  considerable  current 
to  trip  them,  the  contacts  of  the  delicately  constructed  relays 
are  not  heavy  enough  safely  to  close  the  heavy  current  required. 
To  overcome  this  difficulty,  a  relay  switch  as  shown  in  Fig.  195 
is  used.  This  switch  consists  of  a  solenoid  S  with  an  iron 
plunger  P,  to  the  bottom  of  which  is  attached  a  loosely  held  carbon 
disk  C,  insulated  from  the  metallic  plunger.  When  the  overload 
or  the  definite-time-limit  relay  closes  its  contacts,  it  closes 
the  circuit  to  the  relay  switch,  and  the  energized  solenoid 
instantly  pulls  its  plunger  upward,  thereby  pressing  the  carbon 


206 


PROTECTIVE  RELAYS 


FIG.   195. — Relay  switch  with  carbon  contacts. 


FIG.   196. — Relay  switch  with  contacts  at  top. 


MISCELLANEOUS  RELAYS 


207 


disk  C  against  the  two  stationary  carbon  contacts  D  and  D. 
Short-circuiting  these  contacts  closes  the  circuit  of  the  shunt- 
trip  coil  on  the  circuit  breaker.  The  contacts  being  of  carbon 
will  carry  a  heavy  current  and  will  not  stick.  In  another  form 
of  relay  switch,  Fig.  196,  the  plunger  simply  pushes  up  a  pivoted 
arm,  thus  closing  the  two  contacts  D  and  D. 


FIG.   197. — Westinghouse  multi-contact  relay. 

Sometimes  it  is  desirable  to  trip  two  or  more  breakers  at 
once  with  the  same  relay  switch.  In  this  case  the  disk  is 
generally  made  of  copper,  and  two  or  three  sets  of  stationary 
contacts  are  used,  thus  closing  two  or  three  circuits  simulta. 
neously.  Another  multi-contact  relay  is  shown  in  Fig.  197- 


208  PROTECTIVE  RELAYS 

It  must  be  remembered  that  the  arcing  at  the  relay  contacts 
will  always  be  a  great  deal  more  severe  when  opening  a  circuit 
than  when  closing  one.  For  this  reason  a  relay  should  never 
open  the  trip  circuit  once  established.  If  the  trip  circuit  is  fed 
from  the  load  side  of  the  breaker,  it  will  be  opened  automat- 
ically when  the  breaker  opens  and  the  circuit  will  be  dead  when 
the  relays  reset.  Should  it  be  necessary  to  connect  the  shunt 


FIG.   198. — Westinghouse  "transfer"  relay. 

trip  circuit  to  the  line  side  of  the  breaker,  or  if  a  separate  circuit 
is  used,  then  a  switch  must  be  arranged  to  open  the  trip  circuit 
as  soon  as  the  breaker  opens,  thus  relieving  the  relay  contacts 
of  this  duty. 

Transfer  Relays. — To  prevent  the  failure  of  trip  cir- 
cuits as  well  as  to  apply  relays  where  a  direct  current  is  not 
available,  series-trip  or  circuit-opening  relays  may  be  used, 
but  their  inherent  fault  of  opening  a  breaker  on  slight  vibra- 
tion has  discouraged  their  use.  However,  by  using  a  "transfer 
relay, "  as  it  is  called,  the  advantage  of  the  series-trip  relay  may 


MISCELLANEOUS  RELAYS 


209 


be  obtained  without  its  drawback.  Figure  198  shows  one  of 
these  relays  and  Figs.  199  and  200  a  diagrammatic  scheme 
of  parts.  A  is  a  standard  circuit-closing  relay.  The  con- 
tacts on  the  upper  end  of  the  shaft  F  are  arranged  so  that 
C  makes  contact  with  D,  while  E  makes  contact  with  B,  in 
normal  position,  Fig.  199.  When  the  plunger  is  pulled  up 
C  makes  contact  with  B,  while  E  makes  contact  with  D,  Fig.  200. 


Trip     L 
- 


FIG.  199. — Internal  wiring  diagram 
of  transfer  relay. 


FIG.  200. — Showing  positions  of  core 
and  switch  after  tripping. 


The  current  from  the  series  transformer  G  passes  through  the 
relay  A  and  through  two  coils  in  the  transfer  relay.  Coil  H 
tends  to  raise  the  plunger  I  but  coil  J  tends  to  hold  it  down, 
and  since  the  current  in  both  coils  is  equal,  the  plunger  will 
not  be  moved. 

Wound  on  the  same  core  with  coil  J  is  another  coil  K  with 
its  terminals  connected  to  the  relay  contacts.  When  the  relay 
contacts  close  they  short-circuit  coil  K,  which  has  set  up  in  it  a 
current  by  the  transformer  action  of  coil  J.  This  current  being 

14 


210 


PROTECTIVE  RELAYS 


in  opposition  to  the  current  in  coil  J  tends  to  demagnetize  the 
core  M  and  it  loses  its  attraction  for  the  plunger  /,  allowing 
coil  H  to  pull  it  up.  This  changes  the  switch,  at  the  top  to 
the  position  shown  in  Fig.  200.  It  will  now  be  noted  that  the 
current  is  flowing  through  the  trip  coil  L,  which  will  trip  the 
breaker. 

Current 

Transformers 


'Ground 


FIG.  201. — Diagram  of  transfer  relay  connected  on  three-phase  circuit.     Left- 
•    hand  relay  tripped. 


Figure  201  shows  three  single-phase  relays  protecting  a  three- 
phase  line  in  connection  with  three  transfer  relays.  When  the 
plungers  are  all  down,  the  trip  coil  is  entirely  insulated  from 
the  series  circuit.  If  any  one,  two  or  three  relays  operate, 
they  will  complete  a  circuit  through  the  trip  coil  and  current 
transformers  under  any  conditions  that  may  arise.  The  over- 
load relay  A  gives  the  necessary  accuracy,  while  the  transfer 
relay  gives  the  advantage  of  a  series-trip  or  circuit  opening 
system. 

High-tension  Relays. — In  using  an  overload  relay  on  a  high- 
tension  circuit,  it  has  generally  been  customary  to  use  high- 


MISCELLANEOUS  RELAYS 


211 


tension  current  transformers,  which,  of  course,  thoroughly 
insulate  the  relay  circuit  from  the  high-tension  primary  and  allow 
the  relay  to  be  placed  on  the  board.  When  the  current  is  over 
100  amp.  it  requires  only  one  turn  (i.e.,  a  straight  wire)  in  the 
primary  and  consequently  the  insulation  of  the  primary  becomes 
a  comparatively  easy  matter.  Current  transformers  of  one 
turn  are  often  built  right  around  the  terminal  bushings  of  a 
circuit  breaker,  using  the  terminal 
rod  itself  as  the  primary. 

When  the  current  becomes  less 
than  100  amp.,  the  primary  must 
consist  of  more  than  one  turn,  and 
the  insulation  becomes  a  difficult 
matter,  especially  on  an  extra  high- 
tension  circuit,  as  for  instance  a 
66,000-v.  or  a  110,000-v.  line. 

To  overcome  this,  the  complete 
relay  such  as  the  plunger-type  may 
be  mounted  on  a  pillar  insulator,  and 
connected  directly  in  the  high-tension 
line.  Then  if  the  plunger  is  attached 
upward,  due  to  overload,  it  pulls  the 
long,  insulated  chain,  which  me- 
chanically operates  a  set  of  trip  con- 
tacts, thus  closing  the  trip  circuit  to 
the  breaker.  This  chain  is  made  out 
of  micarta  links,  the  number  varying 
according  to  the  potential. 

A  44,000-v.  circuit  should  be  sup- 
plied with  12  links  on  account  of  surges;  a  66,000  with  20  links 
and  a  110,000  with  30  links.  This  allows  the  solenoid  to  be 
mounted  on  a  disconnecting  switch  or  other  insulated  support 
and  the  trip  contacts  to  be  in  the  most  convenient  location. 

Westinghouse  High-tension-relay  Combination. — Realiz- 
ing the  great  advantage  of  an  accurate  high-tension  relay, 
and  the  inability  to  secure  accuracy  with  a  solenoid  type,  the 
high-tension  induction  and  transfer  relay  shown  in  Fig.  202 
was  developed.  It  consists  of  the  accurate  induction-type 
relay  (previously  described)  and  a  transfer  relay  mounted  on 


FIG.  202. — Westinghouse  H. 
T.  current  relay  mounted  on 
post  insulator. 


212  PROTECTIVE  RELAYS 

a  small  panel,  which  in  turn  is  mounted  on  a  pillar  insulator. 
Instead  of  the  transfer-relay  plunger  operating  the  switch, 
it  merely  pulls  the  micarta  chain,  which  closes  the  low-tension 
trip-circuit  contacts. 

The  induction  relay  retains  all  its  inherent  characteristics 
of  inverse,  definite-minimum  time,  its  selective  action,  perma- 
nence of  accuracy,  etc.,  and  the  transfer  relay  adds  the  positive 
tripping  motion,  using  the  A.C.  current  energy  for  operation  and 
not  depending  on  an  auxiliary  trip  circuit.  Together,  they  form 
an  unexcelled  protective  relay  for  high-tension  circuits,  being 
readily  adapted  to  simple  circuits,  radial  systems  and  parallel 
feeders, 

TIMING  RELAYS  WITH  A  CYCLE  COUNTER 

Before  the  introduction  of  the  cycle  counter,  the  generally 
approved  method  of  determining  the  time  delay  of  a  protective 
relay  was  to  use  a  "stop  watch"  or  chronometer,  starting  the 
watch  simultaneously  with  the  application  of  overload  and 
stopping  it  at  the  instant  of  tripping.  With  the  older  relays, 
having  a  time  delay  of  several  seconds,  this  method  gave  satis- 
factory results,  but  with  the  present-day  relays,  designed  with 
watt-hour  meter  accuracy  and  capable  of  being  set  within 
fractions  of  a  second,  it  is  obvious  that  the  stop-watch  method 
is  not  at  all  suitable. 

A  stop  watch  at  best  cannot  be  relied  upon  closer  than  about 
H  sec.,  and  when  to  this  is  added  the  personal  error  of 
starting  and  stopping,  it  gives  a  possible  error  almost  as  great 
as  the  time  between  various  sectionalizing  relays.  It  is  not  at 
all  unusual  to  set  sectionalizing  relays  in  a  radial  system  only 
J^  sec.  apart,  and  in  some  cases  good  results  have  been  obtained 
with  relays  set  only  J£  sec.  apart. 

To  measure  the  time  delay  of  a  protective  relay  accurately 
and  automatically  is  the  function  of  the  cycle  counter.  This 
instrument  is  shown  in  Fig.  203,  while  two  interior  views  are 
shown  in  Figs.  204  and  205. 

Principle  of  Operation. — The  cycle  counter  consists  essen- 
tially of  a  self-winding  clock  in  which  the  escapement  wheel 
or  pendulum  is  replaced  by  a  polarized  relay.  The  diagram- 


MISCELLANEOUS  RELAYS 


213 


matic  scheme  of  parts  is  shown  in  Fig.  206.  The  regular 
escapement  wheel  is  shown  at  W.  Attached  to  this  is  the 
indicating  pointer  moving  over  a  suitable  scale  (as  shown 
in  Fig.  203).  The  escapement  bar  B  allows  the  wheel  W  to 


escape  one  tooth  per  oscillation  in  the  regular  manner.  Rigidly 
attached  to  the  bar  B  is  an  iron  armature,  polarized  by  the 
permanent  magnets  D  and  Dr.  Part  of  the  magnetic  circuit 
is  formed  by  the  two  electromagnets  E  and  Er,  which  are  capable 


214 


PROTECTIVE  RELAYS 


of  attracting  and  repelling  opposite  ends  of  the  armature.     The 
action  is  as  follows: 

Assume  that  during  the  first  cycle  of  applied  current,  the 


-o      ( 

- 

'  — 

^Laminc 
3*  Core 

D-J 

;W 


-     +  MWWVWWWWW 

S3     -fa  fa 1 


FIG.  206. — Schematic  diagram  of  cycle  counter. 
Cycle  Counter 


Relay  Con  tee. 


-  "Electromagnets 
"-Resistor 


•  'Clock  Motor 


Testing 
Switch- 


IIOJ/.A.C. 


FIG.  207. — Connections    for   testing   circuit-closing  relay   with   cycle   counter. 

current  flows  from  the  ±  to  the  +  terminal.  This  will  produce 
an  N-pole  on  the  armature  end  of  the  right-hand  electromagnet 
and  an  $-pole  on  the  corresponding  end  of  the  left-hand  electro- 
magnet. As  both  ends  of  the  armature  are  polarized  N  the 
right-hand  end  will  be  repelled  and  the  left-hand  end  attracted. 


MISCELLANEOUS  RELAYS 


215 


During  the  second  half  of  the  cycle,  the  current  is  reversed, 
and  now  the  right-hand  end  is  attracted  and  the  left-hand 
end  repelled,  which  naturally  results  in  one  oscillation  of  the 
bar  B  and  the  escapement  of  one  tooth.  From  this  it  will 
readily  be  seen  that  the  wheel  moves  one  tooth  per  cycle  as 
long  as  the  electromagnets  are  energized.  A  small  electric 
motor,  controlled  automatically,  re-winds  the  main  clock 
spring,  when  it  has  unwound  a  certain  amount.  This  keeps 
an  even  tension  on  the  escapement,  and  the  electromagnets 
are  thus  not  depended  on  to  drive  the  mechanism,  but  simply 
to  regulate  its  speed. 

Timing  a  Circuit-closing  Relay. — Determining  the  time 
delay  now  becomes  a  problem  of  energizing  the  electromagnets 


Cycle  Counter 


FIG.  208. — Connections  for  testing  circuit  opening  relay  with  cycle  counter. 

simultaneously  with  the  application  of  load  and  de-energiz- 
ing them  upon  the  instant  of  tripping.  This  is  most  easily 
done  by  temporarily  disconnecting  the  relay  from  the  circuit 
and  connecting  to  a  test  circuit  with  connections  as  shown 
in  Fig.  207,  using  a  lamp  bank  or  other  suitable  resistance 
for  the  load  and  a  switch  by  which  this  load  may  be  quickly 
applied.  While  adjusting  the  load,  it  is  best  to  disconnect  the 
lead  at  A  to  avoid  unnecessary  wear  on  the  counter.  Another 
switch  may  be  provided  for  this  purpose  if  desired.  After 
adjusting  the  load,  the  main-testing  switch  is  opened,  the  relay 
allowed  to  reset  fully,  and  the  cycle  counter  pointers  set  on 
zero.  Then  the  switch  is  closed,  thus  applying  load  and  poten- 
tial to  the  relay  and  counter.  The  counter  revolves,  one  tooth  per 


216 


PROTECTIVE  RELAYS 


cycle,  until  the  relay  contacts  close,  when  they  short-circuit  the 
escapement  electromagnets  and  thus  stop  the  counter  instantly. 

The  number  of  cycles  indicated,  divided  by  the  normal  fre- 
quency of  the  testing  circuit,  will  give  the  time  delay  in  seconds. 
For  instance,  if  used  on  a  25-cycle  circuit  and  the  counter  indi- 
cates 50  cycles,  the  time  will  be  2  sec. ;  if  on  a  30-cycle  circuit,  then 
the  time  will  be  1%  sec.,  and  so  on. 

A  number  of  operating  companies  do  not  reduce  the  cycles 
to  seconds,  but  the  testing  reports  and  curves  give  the  time  delay 
directly  in  cycles,  thus  affording  units  which  are  more  easily 
handled  than  fractions  of  a  second. 

Timing  a  Circuit-opening  Relay. — In  determining  the  time 
delay  of  a  circuit-opening  relay,  it  is  simply  necessary  to  con- 
nect the  escapement  solenoids  in  series  with  the  relay  contacts 
as  shown  in  Fig.  208.  Closing  the  main  testing  switch  energizes 
both  relay  and  counter  simultaneously  and  the  counter  stops 
the  instant  the  contacts  open. 


Cycle  Counter 


Transformer 
necessary 


Transformer  if  required, or 

connect  to  other  Transforme 


FIG.  209. — Connections  for  testing  complete  protective  equipment. 

Timing  the  Breaker  or  Oil  Switch. — Realizing  that  it  is 
necessary  to  make  allowance  for  the  time  taken  by  the  breaker 
or  oil  switch  and  its  auxiliary  equipment  to  open  the  circuit, 
it  is  sometimes  preferable,  in  a  closely  set  system,  to  time  the 


MISCELLANEOUS  RELAYS  217 

whole  combination  as  a  unit.  To  do  this,  a  load  must  be 
arranged  so  that  it  can  be  thrown  quickly  directly  on  the  line, 
with  connections  similar  to  those  indicated  in  Fig.  209. 

The  counter  is  connected  to  the  load  side  of  the  switch.  It 
is  energized  at  the  same  instant  that  the  load  is  applied  and  is 
de-energized  the  instant  the  breaker  or  switch  opens. 

This  not  only  gives  the  time  delay  between  instant  of  over- 
load and  opening  of  circuit,  accurately  and  automatically, 
but  also  insures  that  all  protective  apparatus  is  functioning 
properly. 

Typical  Layout. — In  Fig.  210  is  shown  a  typical  layout  which 
may  be  used  to  illustrate  the  uses  of  the  different  kinds  of 
relays. 

There  are  six  generators  shown  which  feed  through  indi- 
vidual switches  to  the  low-tension  bus.  Each  generator  is 
protected  by  relays  shown  at  1  which  may  be  overload,  definite 
limit  relays  of  the  plunger  or  induction  type,  or  reverse-power 
relays  with  current  setting  slightly  less  than  the  sustained 
short-circuit  current  of  the  generators;  or  they  may  be  differen- 
tially-connected overload  relays,  connected  to  trip  the  breaker 
instantly  in  case  of  a  fault  in  the  winding. 

The  bus  is  arranged  to  sectionalize  in  three  sections,  so  if 
any  bus  section  becomes  defective,  the  overload  relays  2,  which 
should  be  of  the  instantaneous  plunger  or  induction  type,  will 
instantly  sectionalize  the  bus. 

The  low-tension  bus  is  arranged  to  feed  four  step-up  trans- 
formers (on  the  left)  and  a  low-tension  radial  system  (on  the 
right).  Each  step-up  transformer  bank  is  protected  by  instan- 
taneous, differentially-connected  overload  relays  4.  Thesfc 
trip  out  an  individual  bank  in  case  of  internal  trouble.  The 
transformers  are  protected  from  overload  by  the  inverse-definite 
minimum-time-limit  relays  of  the  induction  type  shown  at 
3.  The  high-tension  bus  is  also  capable  of  being  sectional- 
ized  by  the  instantaneous  overload  relay  2.  From  this  bus, 
transmission  lines  run  to  two  substations  A  and  B.  Each 
station  is  fed  with  two  parallel  feeders,  protected  at  the  generat- 
ing end  by  overload,  inverse-definite  minimum-time-limit 
relays  5  and  at  the  receiving  end  by  reverse-overload  relays 
6  and  7.  Between  the  two  stations  is  run  another  line,  making 


218 


PROTECTIVE  RELAYS 


a  ring  system.     This  tie  feeder  is  protected  by  reverse-overload 
relays. 

Relays  3  should  be  set  for  about  2  sec.;  relays  5  for  !}•£  sec.; 
relays  6  and  7  for  0.5  sec.;  and  relays  8  for  1  sec. 


S^J7'7Z:/?Y 

FIG.  210. — Indicating  the  use  of  relays  on  typical  power  station. 

The  high  tension  is  stepped  down  by  two  transformers  at 
A.  These  are  protected  by  instantaneous,  differentially-con- 
nected relays  4  and  overload  relays  9  set  low  about  1  sec.  The 


MISCELLANEOUS  RELAYS  219 

low-tension  bus  feeds  three  rotary  converters,  which  feed  a 
three-wire  direct-current  system,  a  storage  battery  and  a  direct- 
current  motor  load.  Relay  10  may  be  an  overload  relay  with 
under-voltage  and  over-voltage  auxiliary  relays.  Relay  11 
should  be  a  high-grade  reverse-current  relay.  Over-voltage, 
overload,  underload  and  under-voltage  relays  may  also  be  used. 
Relay  12  should  be  an  overload  relay  set  about  J£  sec.  or  instan- 
taneous. 

At  substation  B  the  high  tension  is  stepped  down  and  pro- 
tected as  before.  This  secondary  bus  may  feed  a  load  of 
synchronous  motors,  a  three-wire  alternating-current  system  and 
a  constant-current  lighting  circuit.  Relays  13  may  be  overload 
or  reverse  phase.  Relays  14  should  be  overload  with  addi- 
tional under-voltage  and  over- voltage  relays.  Relay  15  should 
be  an  underload  relay. 

From  the  main  station  is  run  a  radial  system  of  parallel 
feeders.  Relays  17,  19  and  21  are  reverse-power  relays,  set 
about  0.1  sec.;  relays  16,  18  and  20  are  accurate  overload 
inverse,  definite-minimum  relays  of  the  induction  type.  Relays 
16  are  set  for  1J^  sec.;  18  for  1  sec.;  and  20  for  %  sec.  At 
each  substation  bus  are  taken  off  a  various  number  of  loads, 
and  each  is  protected  by  relays  which  may  be  of  the  plunger 
type  as  long  as  they  operate  in  quicker  time  than  the  protecting 
sectionalizing  relays. 

It  is  evident  that  no  one  layout  will  suffice  for  all  systems; 
neither  can  invariable  rules  be  laid  out  for  the  use  of  any  relay. 
It  becomes  a  study  of  each  individual  system,  but  with  a  knowl- 
edge of  the  various  loads,  and  how  they  divide  in  the  event  of 
short-circuits,  together  with  the  knowledge  of  the  maximum 
currents,  it  becomes  comparatively  easy  to  apply  relays  and  set 
them  to  give  adequate  protection  and  reduce  unintentional 
interruptions  to  an  almost  negligible  quantity. 


CHAPTER  XV 
TESTING  DIRECT-CURRENT  RELAYS 

No  matter  how  carefully  a  relay  is  constructed  or  tested,  it 
is  always  well  to  remember  that  no  piece  of  apparatus  is  infal- 
lible. For  this  reason,  all  relays  should  be  tested  before  install- 
ing, and  should  be  subjected  to  periodic  tests  after  installation. 

In  well-equipped  meter  shops  will  be  found  adequate  appa- 
ratus such  as  meters,  leads  and  batteries  for  making  simple 
or  elaborate  tests,  as  the  case  may  be.  This  chapter  will,  there- 
fore, treat  mainly  of  tests  made  on  the  relays  while  in  service. 

Installing. — Practically  every  manufacturer  gives  complete 
and  elaborate  directions  for  the  installation  of  his  particular 
type  of  relay,  and  these  instructions,  of  course,  should  be  care- 
fully followed  to  obtain  correct  results.  In  general,  they  deal 
with  mechanical  features  such  as  seeing  that  the  moving  parts 
are  free;  that  there  is  no  dirt  or  packing  material  in  the  relay; 
and  that  there  are  no  loose  screws  or  nuts  or  damaged  parts. 
A  diagram  of  connections  also  accompanies  each  relay  and  this 
should  always  be  used  in  the  absence  of  another  authoritative 
diagram,  which  might  be  used  to  include  other  instruments 
in  the  same  circuit.  A  relay  should  always  be  mounted  on  a 
firm,  solid  support  such  as  a  switchboard.  It  must  be  acces- 
sible for  easy  inspection  and  testing,  and  never  mounted  in  a 
place  where  it  will  be  subjected  to  excesses  in  temperature, 
moisture,  destructive  fumes  or  stray  fields.  In  case  a  relay 
must  be  installed  in  a  dusty  place,  for  instance,  a  flour  mill  or 
cement  mill,  it  must  never  have  open  contacts;  they  must 
always  be  enclosed;  preferably  in  a  glass  cover.  If  a  cover  does 
not  accompany  the  relay,  the  relay  should  be  enclosed  in  a  glass- 
covered  dust-proof  box. 

The  circuit  breakers  and  switches,  too,  must  receive  periodic 
attention  in  order  that  they  should  not  fail  at  a  critical  moment. 
In  fact,  manufacturers  sometimes  recommend  that  the  whole 

220 


TESTING  DIRECT-CURRENT  RELAYS 


221 


protective  combination  of  relays,  relay  switches  and  circuit 
breakers  or  automatic  oil  switches  be  used  to  open  the  circuit 
whenever  it  is  absolutely  necessary  to  open  it,  thereby  insuring 
correct  functioning  of  all  units. 

Ground  Testing. — Before  installing  and  before  making  a 
periodic  test,  the  relay  should  be  tested  for  "  grounds."  While 
this  is  really  a  test  for  live  metal-to-frame  defects,  it  will  readily 
detect  defective  spots  in  the  insulation  which  must  be  rein- 
sulated  against  all  possibility  of  breakdown  in  service.  Figure 
21 1  shows  the  development  of  the  winding  of  a  standard  ground- 


110- Volt,  60  Cycle 
Source 


500  Volts  between  Tapg 


Eubber  Insulation 


FIG.  211. — Small  potential  transformer  fitted  with  dial  switch  and  insulated 
leads  for  testing  purposes.  Lamp  1  indicates  that  the  circuit  is  alive,  lamp 
2  lights  only  on  a  ground  between  testing  terminals. 

test  outfit  capable  of  giving  from  0  to  2,000  v.  in  100-v.  steps. 
It  is  merely  a  small  step-up  transformer  arranged  with  a  primary 
to  connect  to  110-v.  A.C.  and  a  secondary  with  taps.  There  are 
four  taps  with  500  v.  between  adjacent  taps,  and  six  taps 
with  100  v.  between  taps;  by  connecting  to  a  suitable  dial  switch, 
any  voltage  in  100-v.  steps  is  obtained.  In  the  primary  is 
connected  a  snap  switch  and  lamp.  As  a  ground  is  equiva- 
lent to  a  short-circuit  in  this  test,  it  will  light  the  lamp,  thus  indi- 
cating the  defect.  The  other  lamp  merely  tells  if  the  trans- 
former primary  is  energized.  In  the  cover  there  is  also  a  spring 
switch,  so  if  the  cover  of  the  box  is  opened,  as  is  necessary  in  order 
to  change  the  dial  switch,  the  primary  circuit  is  immediately 
opened,  thus  preventing  possible  personal  damage  by  acciden- 
tal contact  with  the  live,  high-voltage  secondary. 

The  test  voltage  used  for  the  current  winding  should  be  about 
twice  the  normal  voltage  of  the  circuit  to  which  the  relay  is 


222 


PROTECTIVE  RELAYS 


connected,  plus  1,000  v.  When  a  separate  trip  is  used,  the  live 
parts  of  the  trip  circuit  should  be  tested  for  grounds.  A  test 
should  also  be  made  between  the  coil  circuit  and  trip  circuit 
terminals.  Figure  212  shows  the  method  of  procedure.  One 
lead  is  held  on  the  frame,  preferably  on  an  unenamelled  screw 
head,  and  the  other  is  touched  to  terminal  A  and  B.  If  nothing 


E     0   C 
FIG.  212. — Ground-testing  a  bellows  type  relay. 

happens,  the  lead  is  then  touched  to  trip  terminals  C,  D  and  E. 
If  this  shows  intact  insulation,  touch  one  lead  to  terminal  A 
and  one  lead  to  C,  D  and  E  in  succession.  If  a  dead  metal-to- 
metal  connection  is  present,  lamp  2  (Fig.  211)  will  light;  other- 
wise nothing  will  happen.  If  due  to  insufficient  insulation, 
the  ground  will  often  show  up  by  a  slight  arc  or  smoke.  The 


0 


c 


A      B      C      D     E      F 
FIG.  213. — Ground-testing  a  D'Arsonval  type  relay. 

relay  cover  should  always  be  on  during  ground  tests,  and  often 
it  is  advisable  to  test  first  with  the  contacts  open  and  then 
again  with  them  closed. 

Figure  213    shows  how  to   test   a   moving  coil  D'Arsonval 
type  relay.     In  this  there  are  three  separate  circuits,  so  it  is 


TESTING  DIRECT-CURRENT  RELAYS  223 

necessary  to  test  between  each  circuit  and  frame,  and  from  each 
circuit  to  the  others.  If  A  and  Fj  B  and  E;  and  C  and  D,  are 
circuit  terminals,  first  touch  one  lead  to  the  case,  and  one  lead 
to  A,  B,  C,  D,  E  and  F  in  succession;  then  touch  one  lead  to  A 
and  one  lead  to  B,  C,  D  and  E  in  succession;  then  one  lead  to  B 
and  one  lead  to  C  and  D. 

Testing  Relay  Switches. — The  relay  switch  is  perhaps  the 
easiest  piece  of  protective  apparatus  to  test.  First  make 
sure  that  all  screws  and  nuts  are  tight;  that  it  is  firmly 
mounted;  no  loose  connections;  and  that  the  contacts  (if  metal) 
are  clean  and  bright,  and  not  burnt. 

If  they  are  burnt  or  pitted,  take  a  piece  of  fine  emery  cloth 
(never  use  crocus  paper,  it  leaves  a  muddy  deposit)  and,  doub- 
ling it,  work  it  back  and  forth  between  the  contacts,  which  should 
be  held  firmly  against  the  emery.  In  this  way,  grind  the  sur- 
faces until  they  meet  accurately. 

Then  energize  the  solenoid  by  connecting  to  a  circuit  of  the 
correct  voltage  and  see  that  the  plunger  rises  freely  and  quickly 
and  closes  the  contacts  positively  and  firmly.  If  the  trip 
circuit  is  operated  from  a  storage  battery,  it  is  well  to  see  that 
the  plunger  rises  satisfactorily  when  the  solenoid  is  energized 
on  70  per  cent  and  130  per  cent  of  the  normal  voltage.  This 
is  to  insure  correct  operation  no  matter  how  low  or  high  the 
battery  voltage  may  go.  Failure  to  operate  should  be  care- 
fully investigated  and  the  cause  removed.  It  may  be  due 
to  foreign  substances,  bent  parts  or  rubbing  magnetic  surfaces. 

Various  Testing  Loads. — If  a  D.C.  circuit  is  available,  and  the 
relay  to  be  tested  is  of  the  millivolt  type,  a  portable  lamp  bank 
or  a  load  box  as  shown  in  Fig.  214  may  be  used  with  connections 
as  shown  in  Fig.  215.  If  undesirable  to  trip  the  breaker  in 
making  the  test,  substitute  a  lamp  as  shown. 

Loads  for  Series-type  Relays. — The  series-type,  such  as  the 
plunger,  relays  are  not  so  convenient  to  test,  as  the  whole  load 
must  be  passed  through  the  series  coil.  If  at  all  possible, 
this  is  done  by  building  up  the  load  until  the  breaker  trips. 

This  test  can  often  be  made  at  night  when  an  occasional 
interruption  does  not  harm  the  service.  If  the  load  is  large, 
water  rheostats  may  be  used.  A  large  barrel,  filled  with  water, 
with,  an  electrode  at  the  top  and  bottom  will  handle  consider- 


224 


PROTECTIVE  RELAYS 


able  load.  The  variation  is  obtained  by  continually  adding 
salt,  meanwhile  watching  the  ammeter  and  noting  the  point 
at  which  the  relay  trips.  As  an  illustration  of  how  great  a 


FIG.  214. — Typical  resistance  load  boxes. 

current  may  be  controlled  in  this  manner,  a  barrel  3  ft.  deep 
with  electrodes  2  ft.  square  should  handle  500  amp.  easily 
at  100  v. 


TESTING  DIRECT-CURRENT  RELAYS 


225 


Another  way  of  obtaining  a  load  is  to  immerse  an  iron-wire 
rheostat  of  the  necessary  resistance  in  running  water. 
For  smaller  loads,   and  where  it  is  undesirable  to  interrupt 


1  Lamp 


FIG.  215. — Testing  relay  with  a  separate  source. 


FIG.  216. — Typical  portable  storage  battery.    (Elec.  Storage  Battery  Co.) 

the  load,  a  storage  battery  such  as  shown  in  Fig.  216  may 
be  used  with  a  water  rheostat  or  a  carbon  rheostat.  A  good 
home-made  carbon  rheostat  is  a  wooden  box  filled  with  arc- 
lamp  carbons  and  brass  plates.  The  handle  can  be  screwed 

15 


226  PROTECT  I VE  RELA  YS 

in,  compressing  the  carbons  and  plates  and  lowering  the  resis- 
tance, thus  increasing  the  current. 

A  box  12  in.  long  by  6  in.  wide  by  6  in.  deep  should  carry 
20  amp.  continuously  at  4  v.  and  will  regulate  as  high  as  200 
amp.  for  short  intervals  of  time.  The  regulation  is  made  in 
infinitely  small  steps. 

Leads  with  heavy  spring  clips  on  the  ends  will  be  found  very 
convenient  for  making  quick  connections  and  will  carry  several 
hundred  amperes.  An  excellent  make  of  clip  is  shown  in  Fig.  217. 


FIG.  217.— "Big  Brute"  testing  clip.     (Mueller  Elec.  Co.) 

Testing  a  Millivolt-type  Relay. — Say  for  example  a  millivolt- 
type  relay  must  trip  when  the  load  reaches  800  amp.  but  it  is 
impracticable  to  get  this  load.  First  disconnect  the  leads  at 
the  shunt,  and  the  trip  leads  at  the  relay.  Give  the  usual 
mechanical  inspection,  clean  the  contacts  and  ground  test. 
Note  the  capacity  of  the  shunt;  say  it  is  50  m.v.  at  1,000  amp.; 
therefore,  at  800  amp.  it  would  give  40  m.v.  (1,000  :  50  =  800 
:  x).  Instead  of  saying  that  the  relay  must  trip  at  800  amp. 
we  can  say  that  it  must  trip  on  40  m.v.;  then  it  simply  becomes 
a  problem  to  obtain  40  m.v.  from  an  external  source.  Take 
a  standard  shunt  giving  50  m.v.  at  5  amp.  and  connect  in  series 
with  the  testing  load.  Gradually  increase  the  load  until  the 
milli voltmeter  reads  40  m.v.  and  adjust  the  relay  so  it  trips. 
Several  trials  may  be  necessary.  Note  that  the  current  is 
unknown  exactly  but  that  it  takes  approximately  4  amp.  in  the 
shunt  to  produce  exactly  the  same  effect  on  the  relay  as  800 
amp.  in  the  station  shunt.  Then,  reconnecting  the  relay  to  the 


TESTING  DIRECT-CURRENT  RELAYS 


227 


station  shunt,  we  know  it  will  trip  on  800  amp.  although  we 
only  tested  it  with  a  4-amp.  load. 

Testing  Plunger -type,  Instantaneous-trip  Relays. — Give  the 
relay  a  thorough  mechanical  inspection,  carefully  trying  every 
nut  and  screw  and  looking  for  burnt  coils  and  loose  con- 
nections; clean  the  contacts  and  then  ground-test.  If  the 


FIG.  218. — Connections  to  boost  or  buck  the  station  load. 

current  capacity  is  low,  say,  below  25  amp.,  it  may  be  pref- 
erable to  connect  a  jumper  around  the  relay  to  complete  the 
circuit  and  then  disconnect  the  relay  from  the  circuit,  recon- 
necting it  to  the  test  load.  If  of  a  larger  capacity,  it  is  preferable 
to  build  up  the  load  on  the  circuit  itself,  if  this  can  be  done 
without  interfering  with  the  service.  Otherwise,  connect 
a  storage  battery  and  rheostat  as  shown  in  Fig.  218, 
being  sure  to  include  the  station  shunt  in  the  battery  circuit. 


228  PROTECTIVE  RELAYS 

This  enables  the  main-load  current  to  be  used  in  addition 
to  the  test  current;  or  if  the  load  current  is  already  too  high 
for  the  lower  settings,  reverse  the  battery  and  buck  the  load 
current  down. 

For  instance,  if  a  relay  must  trip  on  600  amp.  and  the  load 
current  is  only  500  amp.,  then  the  battery  must  supply  the 
extra  100  amp.  It  is  not  necessary  to  measure  these  separately, 
but  the  station  ammeter  will  read  their  sum. 

Testing  Time -limit  Relays. — After  making  all  connections 
so  the  load,  or  overload,  can  be  quickly  applied,  the  time  may 
be  determined  with  a  stop  watch.  First  carefully  set  the  rheo- 
stat to  give  the  desired  current,  at  which  current  it  is  desired 
to  take  the  time;  disconnect  and  let  it  fully  reset.  Then  quickly 
apply  the  load  and  press  the  crown  to  start  the  watch;  both 
at  the  same  instant.  Press  again  to  stop  the  watch  when 
the  contacts  close.  The  watch  hand  indicates  in  fifths  of 
a  second  the  time  required  to  close  at  that  particular  current. 
A  third  press  resets  the  watch  for  the  next  trial. 

If  impossible  to  apply  the  load  quickly,  quite  close  results 
can  sometimes  be  obtained  by  building  up  the  load  and  holding 
down  the  plunger  by  hand,  releasing  it  and  snapping  the  watch 
at  the  same  instant. 

By  varying  the  load  and  the  time  settings,  the  time  of  the 
various  combinations  can  be  obtained. 

CURVES  AND  TABLES 

Whenever  relays  are  tested,  the  results  of  the  test  should  be 
permanently  recorded  in  curves  or  tables.  On  relays  without 
time-delay  element,  the  actual  amperes  may  be  plotted  against 
the  setting  as  in  Fig.  219.  To  do  this,  set  the  relay  for  the 
lowest  setting  (say  4  amp.);  then  slowly  raise  the  current 
and  note  the  reading  of  the  ammeter  just  as  the  plunger  rises. 

Say  it  takes  4  amp.  Make  a  dot  where  the  4-amp.  hori- 
zontal and  vertical  lines  intersect.  Change  the  setting  to  5; 
then  it  may  take  only  4.75  amp.  So  make  a  point  where  the 
4.75  horizontal  line  intersects  the  5  vertical.  In  the  same 
way  locate  the  currents  required  for  6,  7  and  8  amp.,  and  draw 
a  curve  through  them. 


TESTING  DIRECT-CURRENT  RELAYS 


229 


Now  suppose  a  relay  is  set  for  4  amp.  and  gives  satisfactory 
service.  Then,  for  some  reason,  it  is  desired  to  have  the  relay 
changed  to  8  amp.  Before  changing  to  the  No.  8  setting,  the 
operator  would  look  up  his  curve  and  see  that  the  relay  took 
only  7.5  amp.  at  this  setting.  This  might  be  satisfactory 
or  it  might  be  necessary  to  readjust  the  relay  to  give  100  per 
cent  accuracy  at  this  point.  In  any  case,  the  curve  has  saved 


7 

8. 

1 

05 

4 

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urve  o 

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>er  : 

f  Rela 

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5                       A                       5                        6                        7                       € 
Sett-ing   on  Relay 

FIG.  219. — Accuracy  curve  of  relay. 

the  operator  from  depending  on  the  relay  to  trip  at  8  amp. 
and  distributing  his  load  accordingly,  only  to  have  it  trip  at 
7.5  amp.,  perhaps  throwing  out  the  whole  system  without  cause. 

Time  limit  relays  require  more  elaborate  curves  and  tables. 
It  is  generally  best  to  plot  the  load  on  a  certain  setting  against 
the  time.  This  requires  a  separate  curve  for  each  setting, 
although  these  are  frequently  made  on  the  same  sheet. 

Curves  are  generally  to  be  preferred  to  tables  when  in  intel- 
ligent hands,  but  there  is  less  liability  to  error  in  using  tables 
and  expert  opinion  is  divided  as  to  which  is  preferable. 


230  PROTECTIVE  RELAYS 

CONCLUSION 

In  making  very  accurate  tests,  in  which  the  trip  circuit  has 
been  disconnected,  time  must  be  allowed  for  the  closing  of  the 
relay  switch  and  opening  of  breaker.  In  general  this  will  be 
found  to  be  0.2  or  0.3  sec.,  so  when  setting  the  relays  for  actual 
operation,  it  must  be  remembered  that  the  breaker  does  not 
actually  open  until  0.2  or  0.3  sec.  after  the  contacts  close. 

In  setting  time  accurately,  the  type  of  relay  must  be  con- 
sidered and  its  limitations  taken  into  account.  This  is  espe- 
cially true  of  the  bellows  and  dashpot  relays.  The  bellows,  unless 
carefully  oiled  every  few  months,  will  harden  and  the  time  of 
trippings  cannot  be  depended  upon  at  all.  The  dashpot  relays 
will  give  greatly-varying  time  due  to  a  change  in  the  viscosity 
of  the  oil  if  used  where  they  are  subjected  to  extremes  in  tem- 
perature. The  oil  supplied  by  the  maker  should  be  used  to 
the  exclusion  of  all  others. 

When  making  tests,  it  must  be  constantly  borne  in  mind 
that  the  circuit  is  left  without  protection,  and  great  caution 
must  be  used  that  an  overload  does  not  occur  in  the  interval 
of  testing.  Some  modern  boards  have  their  relays  arranged 
with  testing  switches  which  automatically  disconnect  a  relay 
but  connect  in  a  spare  relay  for  protection  during  test.  But 
even  in  this  case,  too  much  caution  cannot  be  used  to  insure 
against  material  damage,  personal  injury  and  avoidable 
interruptions. 


CHAPTER  XVI 
TESTING  ALTERNATING-CURRENT  RELAYS 

Since  protective  relays  use  the  same  principles  of  operation 
as  are  used  in  indicating  instruments,  it  follows  that  they  will 
require  the  same  classes  of  instruments  and  apparatus  to  test 
them.  For  testing  purposes,  the  relays  may  be  divided  into 
three  classes:  Those  requiring  current  alone  for  their  opera- 
tion; those  requiring  voltage  alone;  and  those  requiring  both 
current  and  voltage.  Since  the  majority  of  relays  operate  on 
current  alone,  this  class  will  be  considered  first. 

Relays,  like  every  other  piece  of  mechanical  apparatus,  are 
prone  to  develop  inaccuracies  and  irregularities;  consequently 
a  comprehensive  periodic  system  of  testing  should  be  developed 
and  rigidly  followed  out.  Some  companies  make  it  a  practice 
to  test  relays  every  six  months;  others  test  after  every  dis- 
turbance which  causes  the  relays  to  operate.  This  practice 
is  often  necessary  on  the  bellows-type  relays,  but  is  questionable 
for  the  induction  type. 

RELAYS  REQUIRING  CURRENT  ONLY 

When  considering  the  testing  of  current  relays,  there  are 
five  main  points  to  be  considered.  First,  the  source  of  testing 
supply;  second,  the  load  and  its  regulation;  third,  the  measur- 
ing instruments;  fourth,  the  trip  circuit;  fifth,  the  measurement 
of  the  relay  time  of  operation. 

If  possible,  there  is  only  one  correct  way  to  test  a  relay,  and 
that  is,  to  make  a  dead  short-circuit  on  the  protected  apparatus 
and  see  if  the  relay  operates  and  opens  the  breaker.  However, 
this  is  seldom  possible  or  desirable,  as  it  not  only  throws  a 
heavy  strain  on  all  the  apparatus,  including  the  generators, 
but  entails  a  momentary  interruption  to  the  service.  However, 
an  occasional  accidental  "short"  which  is  properly  cleared 

231 


232  PROTECTIVE  RELAYS 

is  the  best  assurance  that  the  protective  apparatus  as  a  whole 
is  functioning  properly. 

The  Source. — Instead  of  waiting  for  the  accidental  short- 
circuit  to  occur,  it  is  customary  to  subject  the  relays  and  auxil- 
iary apparatus  to  periodic  tests  which  indicate  that  they  are  in 
condition  to  clear  actual  trouble.  Thus,  in  testing,  the  relays 
are  disconnected  from  the  circuit,  and  current  supplied  from 
an  independent  source.  The  trip  is  also  disconnected  from 
the  circuit  breaker,  and  a  signal  lamp  or  cycle  counter  connected 
in  to  determine  the  time.  Since  the  independent  source  of 
testing  supply  need  only  supply  the  losses  of  the  relay,  and 
these  losses  amount  to  only  a  few  watts  (100  at  the  most),  it  is 
customary  to  use  the  ordinary  lighting  circuit  as  the  source. 
Oftentimes,  when  there  are  a  large  number  of  relays  in  one  loca- 
tion to  be  tested,  a  line  or  source  is  conveniently  supplied. 
Of  course,  the  source  must  be  the  same  frequency  as  the  normal 
frequency  upon  which  the  relay  works  and  it  must  first  be 
ascertained  that  the  lighting  circuit  is  not  fed  from  a  storage 
battery  or  spare  unit,  as  is  often  the  case,  in  order  to  provide 
an  unfailing  source  of  light  in  case  all  other  apparatus  fails. 

The  Load. — The  relay  itself  must  now  be  considered.  It 
may  be  of  the  series  type,  in  which  case  the  winding  is  generally 
heavy,  often  being  of  heavy  copper  strap,  wound  on  edge,  and 
in  many  cases,  a  single  bar  passing  through  the  relay.  This 
is  known  as  the  primary  type.  Or  it  may  be  wound  to  operate 
from  the  secondary  of  a  series  transformer.  This  is  known 
as  the  secondary  type  and  the  standard  practice  is  to  use  approxi- 
mately 5  amp.  for  its  operation.  Loading  up  a  primary  relay 
sometimes  becomes  a  difficult  matter,  especially  in  the  larger 
size,  but  loading  a  secondary  relay  becomes  a  very  simple 
matter.  As  a  5-amp.  relay  requires  only  a  few  volts  to  force 
the  necessary  current  through  it,  an  additional  current-limiting 
resistance  or  reactance  is  generally  used  in  series,  if  the  current 
is  to  be  taken  from  a  110-v.  circuit.  This  may  be  a  lamp 
bank,  a  resistance  unit,  a  resistance  or  load  box,  or  a  choke 
coil,  or  reactance  or  impedance  coil.  | 

Reactances  are  seldom  used,  as  the  wave  form  of  their  currents 
is  liable  to  be  peaked,  and  while  present-day  relays  are  but 
slightly  affected  by  distorted  wave  forms,  it  is  undesirable  to 


TESTING  ALTERNATING-CURRENT  RELAYS 


233 


introduce  any  possible  source  of  error.  Resistance  or  load 
boxes  are  often  used.  These  are  arranged  with  a  number  of 
resistance  coils.  All  the  coils  are  connected  together  on  one  side 
but  the  other  ends  pass  through  individual  switches,  which  allow 
any  number  of  coils  to  be  placed  in  parallel.  The  coils  have 
various  resistances,  so  that  each  switch  allows  a  certain  amount 
of  current  to  flow.  For  instance,  when  all  connections  are  made, 


FIG.  220. — Ward  Leonard  resistance  unit. 

and  switch  No.  1  is  closed,  it  allows  J^  amp.  to  flow  through 
the  circuit.  Now  if  switch  No.  1  is  opened  and  switch  No.  2 
closed,  it^may  allow  1  amp.  to  flow.  If  switches  No.  1  and  No.  2 
are  both  closed,  it  will  allow  1H  amP-  Switches  No.  3,  No.  4, 
etc.,  may  control,  2,  2,  5,  10,  20,  30,  etc.,  amp. 

Thus  any  amount  of  current  up  to  the  full  capacity  of  the 
box  may  be  controlled  in  J^  amp.  steps. 

Ordinary   incandescent   lamps  are   used    widely    for  testing 


S.P.  D.  T.  KNIFE  SWITCHED 


FIG.  221. — Very  flexible  load  composed  of  lamps  and  S.   P.   D.   T.  switches. 

loads,  as  they  are  very  convenient  to  manipulate  and  replace. 
Instead  of  lamps,  however,  it  is  more  desirable  to  use  resistance 
units  such  as  are  shown  in  Fig.  220.  Thus  the  advantages 
of  flexibility  and  ease  of  replacement  are  retained,  and  as  the 
units  are  not  easily  broken,  they  may  be  transported  more 
readily  than  lamps. 

A  very  flexible  arrangement  of  lamp  sockets  and  single- 
pole,  double-throw  switches  may  be  easily  constructed  as  shown 
in  Fig.  221.  This  shows  six  units  in  screw  sockets,  all  connected 


234  PROTECTIVE  RELAYS 

in  series.  Between  each  unit  and  at  the  ends  is  connected  a 
single-pole,  double-throw  knife-switch.  Now  if  the  first  switch 
is  placed  up,  the  second  down  (as  shown  in  Fig.  221),  then  the 
first  unit  is  connected  directly  across  the  two  terminals.  If 
the  third  switch  is  placed  up,  then  the  third  unit  is  placed  in 
parallel.  Then  a  switch  may  be  left  open,  and  say  the  fifth 
switch  is  placed  down.  Then  units  3  and  4  are  in  series,  but 
the  two  in  series  are  in  parallel  with  1  and  2. 

Thus  it  is  evident  that  this  arrangement  permits  of  a  varia- 
tion from  all  in  parallel  to  all  in  series,  with  any  combination 


FIG.  222. — Carbon  rheostat. 

of  series-parallel.  If  100-ohm,  100-watt  units  are  used,  the  re- 
sistance of  the  arrangement  may  be  varied  from  16%  ohms 
to  600  ohms,  or  in  terms  of  current  on  a  100-v.  circuit,  from 
6  amp.  to  0.16  amp. 

For  heavy  loads,  a  water  rheostat  is  often  used.  This  merely 
consists  of  two  metal  plates,  placed  in  a  pail  or  barrel  of  acidu- 
lated or  salted  water.  The  current  strength  is  varied  either 
by  varying  the  distance  between  the  plates  or  by  varying  the 
density  of  the  solution.  In  this  type  the  plate  is  lowered  into 
the  solution,  thus  presenting  more  and  more  active  surface 
and  increasing  the  resultant  current  accordingly. 

Carbon  Rheostat. — Another  form  of  load,  which  is  quite 
widely  used,  is  the  carbon  compression  rheostat.  This  utilizes 
the  varying  resistance  produced  between  a  metal  and  carbon 


TESTING  ALTERNATING-CURRENT  RELAYS 


235 


under  varying  pressure.     A  very  useful  carbon  rheostat  is  shown 
plainly  in  Fig.  222. 

Slide  Resistor. — Oftentimes,  due  to  a  variable  line  voltage, 
the  proper  current  cannot  be  obtained  exactly.  To  overcome 
this,  a  very  fine  variable  resistance  is  employed  similar  to 
Fig.  223.  This  is  an  insulated  tube,  wound  with  resistance 
wire,  and  arranged  with  a  "slider"  which  may  be  moved 
back  and  forth,  thus  cutting  in  or  out  resistance  in  very 
small  steps.  This  resistance  may  be  connected  in  parallel 
with  the  load,  but  the  general  practice  is  to  connect  a  slide  of 


FIG.  223. — Ohmic  slide  resistor. 

about  200  ohms  in  series  with  another  resistance  of  50  or  100 
ohms,  thus  limiting  the  current  to  a  total  variation  of  about 
1 J^  amp.  and  obtaining  larger  variations  on  the  main-load  box. 

Phantom  Loads. — The  voltage  required  to  force  current 
through  the  relay  windings  is  very  low,  amounting  to  less  than 
5  v.  in  many  cases.  If  current  is  drawn  from  a  110-v.  circuit, 
by  a  series  load,  it  is  evident  that  only  5  v.  is  actually  used, 
leaving  105  v.  to  be  wasted  in  forcing  current  through  the  load. 
Instead,  however,  of  using  5  amp.  at  110  v.  merely  to  obtain 
5  amp.  in  the  relay,  we  may  transform  from  110  to  5  v.,  and  will 
get  5  amp.  at  5  v.  from  the  secondary,  while  the  primary  only 
supplied  the  same  watts  plus  the  primary  losses  at  110  v. 
The  figure  5  was  only  assumed  in  the  foregoing  cases  to  illus- 
trate the  necessity  of  secondary  voltage.  In  actual  practice, 
the  voltage  is  neither  known  nor  desired  to  be  known  as  it  is 
varied  until  it  produces  the  proper  current  which  alone  must 


236  PROTECTIVE  RELAYS 

be  measured,  regardless  of  whether  the  voltage  is  1  or  2  or  8 
or  10  v.  This  is  called  a  phantom  load,  as  it  permits  of  a  large 
testing  current  and  still  draws  only  a  small  current  from  the 
testing  supply.  This  will  be  given  further  consideration  under 
testing  connections.  Typical  phantom  loads  are  shown  in 
Fig.  224. 


FIG.  224. — Typical  phantom  load  boxes.     (States  Co.) 

Standards. — The  standard  instrument  for  testing  current 
relays  is  an  ammeter.  This  should  be  a  high-class  instrument, 
having  reliable  accuracy,  rugged  in  construction,  dead-beat 
(i.e.,  when  current  is  applied,  it  should  not  overswing  the  mark, 
but  indicate  it  quickly  without  oscillation  or  vibration).  It 
should  be  correct  on  wide  variations  in  frequency,  and  have 
large,  open  divisions  making  it  easily  read.  The  selection 
of  an  ammeter  is  a  proposition  demanding  careful  consideration 
from  a  great  many  points  of  view. 

The  induction  principle  is  generally  conceded  to  be  the  best 
for  an  all-around  good  instrument,  and  an  exponent  of  the  induc- 
tion type  is  shown  in  Fig.  225.  This  embodies  all  the  above 
enumerated  points,  and  will  give  good  service  if  handled  cor- 
rectly. This  ammeter  is  arranged  with  two  capacities.  For 
instance,  if  the  two  links  span  the  two  outside  posts,  marked 
"10  amperes"  as  in  the  illustration,  the  pointer  deflects  to 
full  scale  on  10  amp.,  but  if  the  links  span  the  two  posts  marked 
"5  amperes"  then  full-scale  deflection  is  obtained  on  5  amp. 

When  the  relays  to  be  tested  are  all  about  the  same  capacity 
and  are  120  amp.  or  less,  it  is  possible  to  get  an  ammeter  which 
reads  full  scale  on  the  desired  current.  For  instance,  instead 


TESTING  ALTERNATING-CURRENT  RELAYS 


237 


of  5  and  10  amp.,  it  might  be  10  and  20;  or  20  and  40;  or  40  and 
80;  or  60  and  120. 

Current  Transformers. — Much  greater  flexibility  is  obtained 
by  using  a  5-  and  10-amp.  standard  and  a  current  transformer 
with  variable  ratio.  The  primary  is  connected  in  series  with 


FIG.  225. — Westinghouse  portable  A.C.  ammeter. 
FIG.  226. — Westinghouse  plug  type  transformer. 
FIG.  227. — Westinghouse  through  type  transformer. 


the  line  and  the  current  in  the  secondary  is  multiplied  by  the 
ratio  of  transformation  to  get  the  true  current.  A  well-known 
transformer  is  shown  in  Fig.  226.  This  has  primary  values 
of  25,  50  and  100  amp.,  which  are  changed  by  rearranging  the 
plugs  according  to  directions  which  are  furnished  with  the  trans- 


238  PROTECTIVE  RELAYS 

former.  For  higher  current,  the  transformer  in  Fig.  227  is  used. 
Two  ranges  may  be  supplied,  400  amp.  and  1,600  amp.  The 
primary  is  formed  merely  by  passing  the  wire  or  cable  through 
the  hole.  The  ammeter  is  connected  to  the  secondary  terminals. 
By  passing  two  turns  through  the  hole,  the  ratio  is  changed 
from  400:5  to  200:5  or  from  1,600:5  to  800:5  according  to  the 
transformer  used.  Four  turns  change  the  ratios  to  100:  5  and 
400: 5  respectively.  If  the  current  is  greater  than  1,600  amp.,  the 
10-amp.  range  on  the  meter  is  used,  making  it  possible  to  measure 
up  to  3,200  amp.,  using  the  same  ratio  or  rather  3,220:10.  There 
is  a  slight  error  when  used  in  this  manner. 

The  Trip  Circuit. — The  simplest  way  to  test  the  trip  circuit 
which  operates  from  a  separate  source  is  to  disconnect  the 
leads  from  the  breaker  and  substitute  a  lamp.  If  this  is  imprac- 
ticable, the  leads  should  be  disconnected  from  the  trip  circuit 
of  the  relay,  and  the  terminals  connected  to  a  source  of  supply 
in  series  with  a  lamp.  If  the  relay  is  circuit-closing  (shunt  trip) 
the  pilot  or  signal  lamp  will  light  when  the  relay  trips.  If  the 
relay  is  of  the  circuit-opening  type  (series  trip)  the  lamp  is 
normally  lighted,  but  goes  out  when  the  relay  trips.  When 
testing  a  shunt-trip  relay,  the  contacts  open  the  circuit  when  the 
relay  resets,  and  may  burn  them  slightly.  Therefore,  use 
a  small  lamp  in  testing,  and  before  the  last  time  the  relay  closes 
clean  the  contacts  thoroughly  with  a  piece  of  fine  emery  cloth 
or  paper.  Never  use  the  red,  crocus  paper,  as  it  often  leaves 
a  muddy  coating  on  the  contacts  which  prevents  good  electrical 
contact.  It  is  good  practice,  after  cleaning  the  contacts,  to 
draw  a  piece  of  ordinary  paper  or  cloth  between  the  contacts 
in  order  to  remove  all  traces  of  emery  dust. 

After  cleaning,  try  the  relay  just  once  to  make  sure  that 
nothing  was  damaged  in  cleaning. 

Timing  the  Relay. — For  ordinary  relays,  except  those  used 
for  sectionalizing,  a  stop  watch  does  very  nicely  for  determining 
the  time.  First,  the  load  is  adjusted,  at  which  the  relay  is  to 
be  timed;  the  relay  is  allowed  to  reset  fully,  and  then  the  watch 
is  snapped  to  start  it  and  the  load  switch  closed  at  the  same 
instant.  The  instant  the  relay  trips  (lights  the  signal  lamp), 
the  watch  is  stopped  and  the  time  noted.  The  third  snap 
resets  the  watch  on  zero,  ready  for  the  next  trial. 


TESTING  ALTERNATING-CURRENT  RELAYS 


239 


For  more  accurate  time,  as  is  required  for  sectional! zing  relays 
on  a  radial  or  ring  system,  it  is  necessary  to  use  a  cycle  counter. 
This  was  described  in  a  previous  chapter. 

THE  ACTUAL  TESTING 

When  all  the  apparatus,  including  load  box,  ammeter  stand- 
ard, trip  lamp,  and  various  leads  and  tools  are  ready,  and  before 
touching  the  relay,  there  are  two  things  to  be  done,  one  of 
paramount  importance  and  one  a  smaller  detail.  First,  short- 
circuit  the  current  transformer  right  at  the  secondary  leads. 
A  current  transformer  must  never  be  open-circuited  when  there 


I 


FIG.  228. — Testing  a  relay  using  a  separate   testing  source  and  trip  circuit. 

is  load  in  the  primary  as  it  not  only  may  harm  the  transformer, 
but  it  may  induce  a  voltage  of  several  thousand  volts,  making 
it  a  source  of  great  personal  danger.  No  harm  can  come  of 
short-circuiting  a  current  or  series  transformer.  Many  modern 
installations  have  a  permanently-mounted  switch  which  short- 
circuits  the  secondary,  but  in  its  absence,  a  short  lead  with  two 
heavy  spring-testing  clips  should  be  used,  and  fastened  so  that 
they  cannot  possibly  drop  off.  The  second  point  is  to  wipe 
off  all  the  dust  or  dirt  from  the  cover  before  removing  it. 
Assuming,  for  the  first  example,  that  it  is  a  5-amp.  bellows 
relay  which  is  to  be  tested.  It  operates  from  a  series  transformer 


240  PROTECTIVE  RELAYS 

placed  in  the  high-tension  line.  A  separate  source  is  avail- 
able for  testing  and  a  lamp  load  is  used.  The  transformer 
is  short-circuited;  the  relay  disconnected  at  both  top  and  bottom; 
the  load,  ammeter,  relay  coil  and  switch,  all  connected  in  series 
across  the  line;  and  the  trip  connected  in  with  a  signal  lamp,  as 
in  Fig.  228. 

Of  course,  if  the  circuit  may  be  interrupted,  it  is  not  nec- 
essary to  disconnect  the  trip;  the  breaker  itself  actually  may 
be  opened  when  the  relay  trips.  This  is  really  preferable,  but 
often  impossible  to  do.  The  switch  is  closed,  the  load  adjusted 
until  the  meter  shows  the  correct  current  passing;  then  the  switch 
is  opened  and  the  relay  fully  reset.  Quickly  closing  the  switch, 
snap  the  stop  watch  to  start  in,  and  the  instant  the  lamp  lights 
(or  the  breaker  trips),  snap  the  watch  to  stop  it.  This  gives  the 
time  of  delay  between  overload  and  tripping. 

The  Cycle  Counter. — To  obtain  very  accurate  time,  as  is 
necessary  with  sectionalizing  relays,  a  cycle  counter  is  connected 
in,  to  automatically  time  the  delay.  The  self-winding  clock 
is  permanently  connected  across  the  source  of  supply.  The 
escapement  magnet  is  connected  in  series  with  a  resistor,  and  is 
energized  as  soon  as  the  switch  is  closed.  Then  it  starts  count- 
ing the  cycles,  until  the  relay  contacts  close,  thereby  short- 
circuiting  the  escapement  magnets  and  stopping  the  counter 
instantly.  The  cycle  counter  then  indicates  the  number  of 
cycles  which  have  elapsed  between  the  instant  of  load  and  closing 
of  contacts. 

Dividing  this  number  by  60  gives  the  number  of  seconds 
on  a  60-cycle  circuit,  and  dividing  by  25  gives  the  seconds  on 
a  25-cycle  circuit. 

If  a  circuit-opening  relay  is  used,  the  escapement  magnet 
is  placed  in  series  with  the  contacts  with  one  switch  control- 
ling both  load  and  trip.  Then  the  counter  starts  when  the  switch 
is  closed  and  stops  when  the  switch  is  opened. 

Some  engineers  prefer  to  leave  the  circuit  intact  and  connect 
in  the  testing  load  as  shown  in  Fig.  229.  This  scheme  is  excel- 
lent, provided  the  primary  of  the  current  transformer  is  not 
energized,  and  there  is  no  possibility  of  its  being  short-circuited. 
In  this  case,  the  naturrl  impedance  of  the  transformer  second- 
ary prevents  any  appreciable  current  being  diverted  from  the 


TESTING  ALTERNATING-CURRENT  RELAYS 


241 


relays.  It  will  be  noted  that  other  instruments  in  this  circuit 
will  also  receive  the  test  current.  This  method  will  detect 
any  short-circuit  in  the  transformer  or  wiring,  but  a  separate 
continuity  test  may  be  made  by  opening  the  circuit  and  inserting 
potential  to  see  that  no  open  circuits  have  developed.  This 
test  may  be  made  on  a  few  amperes  D.C.,  but  if  A.C.  is  alone 
available,  it  must  be  remembered  that  full  current  cannot 
be  forced  through  the  transformer  secondary  unless  the  primary 
or  secondary  is  short-circuited. 


FIG.  229.— Testing  a  relay  without  disconnecting  from  the  circuit. 


Testing  Heavy-current  Relays. — The  problem  of  testing 
a  heavy-current  relay  is  more  difficult  of  solution,  as  it  is  neces- 
sary to  interrupt  the  circuit  in  most  cases,  unless  the  load  can 
be  built  up  on  the  lines.  In  this  case,  however,  it  is  very  difficult 
to  get  an  accurate  time  as  it  is  hard  to  apply  a  heavy  load  in 
a  quick,  accurate  manner. 

A  good  way  to  test  series  relays  up  to  several  hundred  am- 
peres is  to  use  a  phantom  load,  obtained  by  using  an  old  series 
transformer  inverted.  The  connections  are  shown  in  Fig. 
230.  The  relay  is  disconnected  from  the  main  line.  (This 
is  not  necessary  if  it  is  a  low-tension  circuit  and  if  it  has  no 
current  flowing.)  A  short  section  of  heavy  cable  is  run  through 
two  transformers  A  and  B  and  connected  to  the  relay  coil  F* 
Current  is  now  fed  into  the  secondary  of  transformer  A  with 
load  D  and  switch  E  in  series.  This  current  induces  a  voltage 
in  the  loop  of  wire,  and  since  its  resistance  is  low,  a  heavy  current 

16 


242 


PROTECTIVE  RELAYS 


flows  in  the  relay  circuit.     This  is  measured  accurately  by  the 
transformer  B  and  the  meter  C. 

For  instance,  using  transformers  400  to  5  amp.,  when  meter 
C  reads  5  amp.,  it  will  be  known  that  there  is  400  amp.  in  the 
relay  circuit.  It  may,  however,  take  about  6  amp.  from  the 
line  to  produce  this  current.  Should  one  turn  in  transformer 
A  not  give  enough  current,  the  turns  may  be  increased  to 


•...—Main  Line - 


^Disconnect  at  nearest 
Joint 


'Disconnect  at  nearest  Joint 


— Main  Line- 


FIG.  230. — Using  an  inverted  series  transformer  to  supply  a  heavy  "phantom" 

current. 

two  or  three  and  the  current  from  the  load  increased  accord- 
ingly. A  good  combination  for  400  amp.  is  to  use  a  l,600-to-5 
transformer  at  A  with  four  turns  of  heavy  wire  for  the  primary. 

The  trip  circuit  may  be  connected  with  a  lamp  or  cycle 
counter  as  previously  described. 

Making  the  Adjustment. — There  are  so  many  different  makes 
of  relays  and  principles  of  operation  that  it  is  impossible  to 
give  explicit  directions  for  the  adjustment  of  each  and  every 
one.  This,  however,  is  hardly  necessary,  as  each  manufacturer 
gives  directions  for  readjusting  his  particular  relay  and,  of  course, 
these  should  be  followed.  There  are,  in  general,  two  separate 


I 
TESTING  ALTERNATING-CURRENT  RELAYS  243 

objects  to  be  obtained  by  adjustment — starting  current  and 
time  delay  on  a  definite  current. 

In  the  plunger-type  relays,  the  starting  current  is  adjusted 
by  varying  the  position  of  the  core  on  the  stem.  This  stem  is 
generally  threaded,  and  the  core  or  plunger  screwed  into  position 
and  locked  with  a  setscrew  or  locknut.  When  adjusting,  loosen 
this  set,  then  raise  the  plunger  (screw  it  up)  to  lower  the  start- 
ing current  or  lower  it  to  raise  the  amount  of  starting  current. 
When  the  correct  position  is  found,  lock  the  plunger  firmly 
in  position  so  that  the  excessive  vibration  found  in  this  type 
of  relay  cannot  jar  it  loose.  If  there  are  springs  which  tend 
to  offset  the  weight  of  the  plunger,  these  may  also  be  called 
into  play  to  change  the  starting  current. 

In  the  induction  type,  the  adjustment  is  generally  done  by 
pulling  out,  or  letting  in,  some  of  the  spiral  spring  which  resists 
the  turning  effort  of  the  disk.  This  is  a  delicate  job  and  should 
only  be  undertaken  by  an  experienced  instrument  man.  Slight 
adjustment  can  sometimes  be  made  by  changing  the  position 
of  the  torque  compensator  with  respect  to  the  base.  This 
changes  the  amount  of  leakage  flux  as  the  base  diverts  more 
or  less  of  it,  but  this  changes  the  shape  of  the  curve  more  than 
the  starting  current. 

The  time  of  the  bellows  and  oil-lagged  relays  is  varied  both 
by  the  distance  of  contact  travel  and  the  size  of  the  opening 
in  the  needle  valve.  The  contact  disk  is  generally  placed 
loosely  between  two  nuts  on  the  threaded  shaft  and  may  be 
raised  or  lowered,  thereby  making  the  plunger  travel  a  longer 
or  shorter  distance  and  giving  a  longer  or  shorter  time  delay. 
To  obtain  larger  variations,  it  is  necessary  to  grind  a  flat  place 
on  the  valve,  to  allow  greater  escape  of  air  or  oil  on  long-time 
settings.  Care  must  be  taken,  however,  that  the  valve  is  not 
ground  too  much.  It  is  preferable  not  to  touch  the  valve, 
but  to  make  a  curve  or  set  of  readings,  and  set  the  relay  time 
according  to  this  curve. 

The  time  of  induction-type  relays  is  varied  by  moving  the  per- 
manent magnets  in  or  out. 

Moving  the  magnet  in  toward  the  center  of  the  disk  gives  a 
shorter  time  and  moving  it  out  (tc  within  J^  in.  of  the  edge) 
gives  a  longer  time.  Great  care  must  be  taken  that  the  disk 


244  PROTECTIVE  RELAYS 

docs  not  touch  the  magnet  as  it  turns,  and  that  there  is  no  iron 
dust  or  filings  on  the  magnets  which  may  in  time  rub  on  the  disk. 
A  feather  or  a  piece  of  soft-iron  wire,  (such  as  an  unbent  paper 
clip),  are  excellent  in  removing  filings. 

Before  leaving  a  relay,  make  sure  that  every  accessible  screw 
and  nut  is  tight;  see  that  the  contacts  are  clean;  no  loose  con- 
nections; no  burnt  coils  or  insulation;  no  dirt  or  dust;  and  that 
all  moving  parts  move  freely  without  sticking  or  rubbing.  Also 
see  that  the  leather  on  bellows  relays  is  soft  and  pliable,  using, 
if  necessary,  a  little  neatsfoot  oil  to  keep  it  soft.  Make  sure 
that  all  connections  are  returned  exactly  as  found.  If  there  are 
several  connections  to  be  removed  and  there  is  any  possibility 
of  getting  them  twisted,  make  a  sketch  of  the  relays  and  terminals, 
and  number  each  terminal,  1,  2,  3,  etc.,  on  the  sketch.  Then 
as  the  wires  are  removed,  tie  a  small  tag  on  each  one,  marked 
1,  2,  3,  etc.,  so  there  will  be  absolutely  no  danger  but  that  the 
leads  are  correctly  replaced.  Then  after  all  connections  are 
completed,  remove  the  short  on  the  series  transformer.  Connect 
an  ammeter  in  parallel  with  the  current  coil  and  see  by  its  indica- 
tion that  current  is  flowing.  The  ammeter  will  divert  part 
of  the  current  from  the  relay.  Connect  a  voltmeter  across 
the  trip-circuit  terminals  and  see  by  its  indication  that  the  trip 
circuit  is  intact  right  up  to  the  relay  contacts. 

Plotting  the  Results. — The  time  results  under  various  loads 
are  easily  plotted  in  curve  form  or  tabulated,  thus  rendering 
a  permanent  record  of  the  action  of  the  relay  under  various 
conditions  and  forming  a  ready  source  of  reference  by  which 
the  time  of  the  relay  may  be  easily  changed  in  definite  manner, 
as  might  be  necessary  due  to  a  change  in  the  distribution  of  the 
load  or  the  addition  of  various  equipment. 

If  the  results  are  tabulated,  they  should  be  somewhat  similar 
to  the  table  shown  in  Fig.  231.  The  vertical  columns  are  the 
results  of  the  various  lever  settings  and  the  horizontal  lines 
are  the  various  loads.  Proceed  with  the  test  as  follows:  Set 
the  relay  to  No.  1  setting  and  adjust  the  load  to  one  and  one- 
half  times  the  tap  setting.  Thus  if  the  5-amp.  tap  is  used,  the 
current  must  be  7.5  amp.  The  column  may  be  in  actual  current 
or  in  "per  cent  load."  Apply  the  load  and  note  the  time.  It 
is  0.5  sec.  (30  cycles  on  the  cycle  counter  on  60-cycle  circuit). 


TESTING  ALTERNATING-CURRENT  RELAYS 


245 


Put  this  down  at  the  intersection  of  the  No.  1  setting  column 
and  the  "one  and  one-half  times  current  setting "  line.  Try 
No.  2  setting.  The  time  is  now  1.1  sec.  Try  No.  3  setting. 
It  is  now  1.7  sec.  And  so  on  until  all  the  settings  have  been 
tried,  the  load  remaining  at  one  and  one-half  times  or  7.5  amp. 
during  these  10  tests. 

Now  change  the  load  to  two  times  the  current  tap  setting 
and  take  10  more  readings  with  the  different  lever  settings. 
This  gives  the  figures  for  the  second  horizontal  line. 


Relay  Service: 


Loca+ion  : 


Date: 


Voltage  

*-  1J8  Per  Ceni:  Nor  ma/ 

59  PerGen+. 
NorrTiott 

Millivote  SefHng- 

-*-Z 

5 

10 

5 

Applied  Millivotts 

Tripping  77me  in  Seconds 

2 

6.0 

5 

1.4 

8.0 

10 

0.7 

2.0 

8.0 

\_zz 

15 

0.5 

1.? 

?.& 

1.0 

20 

0.4- 

06 

1.6 

0.6 

1.5 
2 

.Times     3 
Currentr 
Tap 
Setting  10 

20 
30 
50 

Time     in     Seconds   to    Trip 

0.5 

1.  1 

1.7 

2.3 

2.9 

3.7 

4.7 

5.9 

7.2 

8.6 

0.4 

0.8 

1.1 

1.5 

2.0 

2.5 

3.1 

3.9 

4.6 

5.6 

0.3 

0.6 

0.8 

t.l 

1.4 

1.8 

2.2 

2.7 

3.2 

4.0 

0.3 

0.5 

0.7 

0.9 

I.I 

1.4 

1.7 

2.0 

2.5 

3.0 

0.3 

0.4 

0.6 

0.7 

0.9 

1.1 

1.3 

1.7 

2.0 

24 

0.2 

0.4 

0.5 

0.7 

0.8 

1.0 

1.2 

1.5 

1.8 

2.2 

0,2 

0.3 

0.5 

0.6 

0.8 

0.9 

1.1 

f.4 

1.7 

2.0 

0.2 

0.3 

0.4 

0.6 

0.7 

0.8 

1.0 

1.3 

1.6 

1.9 

st^TSq       t       2      3       4       5       G       7       8       9 

10 

FIG.  231. — Tables  of  relay  accuracies. 

When  getting  heavier  loads,  say  above  20  times  or  above 
100  amp.,  especially  on  the  high-lever  settings,  be  sure  the  wind- 
ing is  thoroughly  cooled  off  before  making  the  next  test  as  some- 
times a  repeated  heavy  overload  will  change  the  time  due  to 
the  self-heating. 

If  preferred,  a  curve  may  be  plotted  on  cross-section  paper 
as  shown  in  Fig.  232.  The  point  of  intersection  between  the 
current  and  lever  setting  is  found  and  a  dot  marked.  When 
a  number  of  points  have  been  located,  a  smooth  curve  is  drawn 
through  the  points.  A  table  like  Fig.  231  is  generally  made 


246 


PROTECTIVE  RELAYS 


first  and  then  the  curves  plotted.     Intermediate  time  is  easier 
to  locate  on  a  curve  than  on  a  table. 

Another  important  feature  on  a  large  system  is  the  keeping 
of  a  card  system  giving  complete  data  on  each  relay  together 
with  its  actual  time  and  load  setting.  The  layout  of  the  system 
will  give  the  identifying  number  on  each  relay  on  each  circuit 
and  then,  by  referring  to  this  number  in  the  card  index,  the  per- 


A=  Contacts  set  for  ?  MilUvotts  trip,  US  Per  Cent.  Normal  VoVts 
B=        '»  »      «    5        *  wnnw          «  v 

B1"      «          «»»5*  »     59  »»      *         »  « 

FIG.  232. — Curves  of  first  table  in  Fig.  231. 

f  ormance  of  this  relay  under  certain  combinations  of  load  condi- 
tions can  be  accurately  forecast.  This  also  forms  an  accurate 
method  of  retiming  a  system  in  case  of  revisions  or  additions 
to  the  load. 

TESTING  VOLTAGE  RELAYS 

The  apparatus  generally  used  for  testing  voltage  relays,  both 
under  and  over,  is  a  small  portable  transformer,  a  means  of 
varying  it,  and  a  reliable  standard.  Of  course,  if  a  high  enough 
source  is  available,  it  is  unnecessary  to  have  a  small  step-up 
transformer,  but  only  means  of  cutting  the  voltage  down  to  the 
proper  amount.  A  small  portable  transformer  is  shown  in  Fig. 
233.  While  designed  to  step  down  from  400  or  200  to  100  v., 
with  accurately  compensated  ratio,  it  may  also  be  used  to  step 
up  from  100  to  200  or  400  v.  with  good  results.  The  potential 
is  varied  by  means  of  the  adjustable  slide.  This  slide  is  con- 


TESTING  ALTERNATING-CURRENT  RELAYS 


247 


nected  directly  across  the  line,  while  the  potential  is  taken  be- 
tween one  line  and  the  slide,  thereby  insuring  a  close  graduation 
from  zero  to  maximum  voltage. 

One  form  of  a  reliable  standard  voltmeter  is  of  the  moving 
coil,  dynamometer  type,  which  is  without  question  the  best 
principle  of  operation  to  use  for  an  accurate,  reliable,  rugged 
test  instrument.  Voltmeters  come  with  two  or  more  ranges,  i.e., 
they  indicate  full  scale  on  150  v.,  or  by  changing  to  another 


FIG.  233. — Westinghouse  portable  voltage  transformer. 

terminal,  they  indicate  full  scale  on  300  v.;  or  on  300  and  600. 
A  good  combination  is  a  75-  and  150-v.  voltmeter,  with  an  exter- 
nal multiplier,  making  full  scale  300  v.  and  600  or  750  v. 

In  approaching  an  installation,  the  first  thing  to  do  is  to 
disconnect  the  trip  circuit  and  then  open-circuit  the  potential 
transformer.  Never  short-circuit  it,  (as  in  the  case  of  a  series 
transformer),  as  it  will  invariably  blow  a  primary  fuse  or  burn 
up  the  transformer.  Then  clean  the  dust  and  dirt  from  the  cover 
before  opening  it. 

Connect  the  apparatus  as  in  Fig.  234.  Inspect  the  relay 
mechanically,  tightening  all  screws  and  nuts;  clean  the  contacts 


248 


PROTECTIVE  RELAYS 


and  see  that  all  moving  parts  are  free  to  turn  without  rubbing 
or  friction;  see  that  the  springs  are  intact,  no  burnt  coils,  and 
no  filings  clinging  to  the  magnet. 

Then  close  the  test  switch  and  see  that  the  contacts  close 


FIG.  234. — Testing  a  voltage  relay  from  a  separate  source. 

positively  at  the  right  voltage.  If  desirable,  a  table  or  curve 
may  be  made  showing  the  relation  between  lever  settings  and 
closing  or  opening  volts.  Or  a  curve  may  be  made  showing 
the  length  of  time  required  to  close  the  contacts.  Another  set 
of  test  connections  are  shown  in  Fig.  235. 

^=^_^ 

LINE 


BOOSTING  TRANSFORMER 
FOR  OVERVOLTA&E 


• VAVWWv* 

RHEOSTAT 


-STANDARD 
VOLTMETER 


FIG.  235. — Connections  for  testing  a  voltage  relay. 

Power-directional  Relays. — Tests  on  a  reverse  power  relay 
generally  comprise  a  complete  test  on  the  overload  element 
the  same  as  described  previously,  and  a  thorough  mechanical 
inspection  is  given  to  see  that  all  parts  are  free  to  move  without 


TESTING  ALTERNATING-CURRENT  RELAYS  249 

friction.  In  spite  of  all  calculations  of  overload  currents,  dis- 
torted phase  angles  and  other  effects,  there  is  only  one  sure  way 
of  determining  if  a  certain  installation  of  relays  will  protect, 
and  that  is  by  actually  overloading  or  short-circuiting  the  lines, 
or  in  intentionally  producing  the  disturbance  which  it  is  desired 
to  have  cleared  in  case  of  its  accidental  occurrence.  These  are 
really  installation  tests  and,  once  made,  need  not  be  repeated, 
unless  conditions  occur  which  necessitate  a  revision  of  lines 
or  apparatus.  If  it  be  known  that  the  relays  and  connections 
give  protection  (from  actual  test)  then  it  only  becomes  neces- 
sary to  know  that  the  instruments  are  mechanically  intact  and 
this  is  determined  by  periodic  inspection. 

In  testing  the  directional  element,  both  voltage  and  current, 
should  be  supplied  to  see  that  the  relay  functions  correctly  on 
reversal  of  current.  In  many  cases  it  is  desirable  to  step  the 
voltage  down  to  1  or  2  per  cent  of  normal  and  put  on  a  heavy 
overload  at  low  power  factor  and  see  if  operation  is  satisfactory. 

Reverse-phase  Relays. — Reverse-phase  relays  are  tested 
in  the  same  manner  as  voltage  relays,  except  that  two  circuits 
are  used.  After  the  usual  mechanical  inspection,  one  phase 
is  disconnected  and  the  relay  should  trip;  or  if  the  voltage 
drops  to  a  certain  value  it  should  trip;  or  if  either  phase  is  reversed, 
the  relay  should  trip. 

Temperature  Relays. — Wherever  possible,  the  temperature 
relays  should  be  tested  by  packing  a  thermometer  in  or  near  the 
exploring  coil  and  noting  if  the  indications  correspond  to  the 
relay  action.  On  some  types,  it  is  possible  to  remove  the  explor- 
ing coil  as  a  unit.  In  this  case,  the  exploring  coil  may  be  placed 
in  a  heated  chamber,  together  with  an  accurate  thermometer 
and  the  relay  and  thermometer  compared  from  time  to  time  as 
the  temperature  changes. 

CONCLUSION 

While  directions  have  been  given  for  the  selection,  installa- 
tion and  care  of  various  relays,  it  must  be  understood  that 
what  applies  to  perfection  in  one  case  may  fail  in  another. 
Consequently,  for  any  given  installation,  it  is  necessary  to 
study  the  system,  the  location  of  apparatus,  their  behavior 


250  PROTECTIVE  RELAYS 

under  varying  conditions,  and  finally  to  devise  a  system  of  protec- 
tion which  will  cover  the  great  majority  of  points,  for  it  is  frankly 
acknowledged  that  every  point  cannot  be  covered  by  any  relay 
or  combination  in  every  case. 

The  subject  of  relays  and  their  protection  of  circuits  and 
apparatus  is  an  engineering  profession  by  itself  and  is  capable 
of  great  possibilities  in  improvement. 

However,  even  with  the  present  relays  and  systems,  they  are 
so  little  understood  that  the  economies  and  improved  service 
resulting  from  the  use  of  relays  have  not  been  taken  advantage 
of  by  operators  as  fully  as  they  should  be.  Then,  too,  many 
operators  have  not  yet  realized  these  advantages.  Others 
that  have  used  relays  to  a  limited  extent  have  in  no  way  exhausted 
their  possibilities. 

The  electrical  engineers  of  the  country  are  still  at  work  on 
the  subject  of  protection  and  uninterrupted  service,  and  are 
gradually  and  thoroughly  investigating  each  and  every  possible 
protection  problem  and  each  year  sees  many  old  problems 
successfully  solved.  So  it  is  to  be  hoped  that  soon  such  things 
as  burnouts  and  short-circuits  interrupting  service  will  be 
matters  of  the  past. 


CHAPTER  XVII 
LOCATING  FAULTS  IN  FEEDERS  AND  WIRING 

Although  protective  relays  may  sectionalize  and  isolate 
defective  feeders  and  apparatus  with  more  than  human  speed  and 
accuracy,  yet  it  is  often  a  difficult  matter  to  locate  the  actual 
fault.  In  the  factory  the  fault  may  be  in  a  conduit  or  duct; 
in  an  underground  distribution  system,  it  may  be  in  a  subway 
between  manholes;  or  in  a  long-distance  transmission  system, 
the  location  of  the  fault  may  require  miles  of  line  patrolling  to 
'actually  see  where  and  what  the  trouble  may  be.  It  often 
happens  that  relays  may  isolate  a.  line  and  then  considerable 
time  is  spent  in  trying  to  locate  the  fault,  where  in  other  cases 
it  may  be  that  the  fault  clears  immediately  upon  isolation. 

The  following  chapter  while  not  a  complete  exposition  on 
the  details  of  fault  location,  especially  as  applied  to  high-tension 
lines,  will  nevertheless  give  the  basic  principles  of  the  methods 
used  in  finding  the  exact  location  of  the  fault. 

The  most  commonly  met  defects  or  faults  in  the  wiring  of 
any  system  are:  (1)  open-circuits,  or  breaks,,  caused  by  a  broken 
wire  or  blown  fuse;  (2)  short-circuits,  or  crosses,  caused  by  two 
metal  conductors  of  different  potentials  touching  each  other; 
and  (3)  grounds,  caused  by  a  live  metal  conductor  touching  a 
metal  conduit  or  other  foreign  metal.  An  open-circuit  is  in 
reality  a  complete  break,  or  condition  of  infinite  resistance 
in  a  supposedly  continuous  conductor,  but  there  are  condi- 
tions when  the  fault  may  have  any  resistance  from  a  few  ohms 
to  many  thousand.  In  such  cases,  however,  the  fault  is  usually 
accompanied  by  a  ground,  as  for  instance,  if  a  feeder  in  a  metal 
conduit  should  become  open-circuit,  the  voltage  might  cause 
the  two  ends  to  weld  fast  to  the  conduit,  thus  introducing  only 
a  slight  resistance  in  the  line  producing  a  ground. 

A  short-circuit  may  be  either  a  low-resistance  or  "dead" 
short,  or  a  high-resistance  short-circuit,  commonly  called  a 

251 


252  PROTECTIVE  RELAYS 

leak.  A  "dead"  short  will  usually  manifest  itself  in  a  violent 
manner  by  blowing  a  fuse  or  breaker,  and  the  fuse  cannot  be 
replaced  or  the  breaker  reset  until  the  fault  is  located  and 
cleared.  A  high-resistance  short  may  not  draw  enough  current 
to  blow  a  fuse,  but  it  constitutes  a  waste  of  current,  and  if  the 
leak  be  confined  to  a  small  space,  the  resulting  heat  may  start 
a  fire.  As  before,  short-circuits  are  often  accompanied  by  open 
circuits  and  grounds,  due  to  the  violence  of  the  short-circuit 
either  burning  the  wire  in  two  or  welding  it  to  the  metal  conduit. 

On  low-potential  circuits,  a  single  ground  cannot  cause  any 
damage;  but  should  a  second  ground  occur  on  a  wire  of  opposite 
polarity,  the  two  grounds  will  cause  a  short-circuit,  its  violence 
depending  on  whether  the  grounds  are  of  high  resistance  or 
low  resistance.  There  are  a  great  many  cases  where  it  is  advis- 
able to  ground  a  machine  frame  or  wire  in  order  to  limit  the 
potential  between  any  part  of  the  circuit  and  the  ground;  for 
instance,  the  secondary  of  a  110-v.  lighting  circuit  is  usually 
grounded  so  that  there  can  never  be  a  dangerous  potential  be- 
tween the  wiring  and  ground  in  case  the  transformer  should 
short-circuit  between  primary  and  secondary.  Another  case  is 
where  the  neutral  of  a  three-wire  system  is  grounded  so  that 
the  potential  between  either  outside  wire  and  the  ground  can 
never  be  greater  than  the  potential  between  either  outside  wire 
and  the  neutral.  In  testing  for  faults  it  must  be  remembered 
that  all  grounds  are  not  accidental;  some  are  intentional  and 
must  be  taken  into  account  in  the  test. 

Apparatus  Required. — Perhaps  the  most  commonly  used  test- 
ing apparatus  is  a  magneto  and  polarized  bell,  such  as  are  often 
found  in  the  old-style  telephones.  This  is  shown  in  Fig.  236. 
The  magneto  and  bell  are  mounted  in  a  compact  portable 
carrying  case,  connected  in  series  and  supplied  with  a  long  pair 
of  leads.  This  outfit  is  used  extensively  to  test  wiring  for  opens, 
shorts  and  grounds  as  will  be  described  later.  A  special  fuse- 
testing  panel  is  shown  in  Fig.  237. 

On  long  lines,  however,  the  magneto  and  bell  test  for  open 
circuits,  or  rather  for  continuity,  must  be  used  with  great  cau- 
tion and  intelligence  as  often  there  will  be  enough  capacity 
or  condenser  effect  to  allow  the  bell  to  ring  even  though  there 
be  an  open  circuit. 


LOCATING  FAULTS  IN  FEEDERS  AND  WIRING 


253 


The  testing  apparatus  shown  in  Fig.  238  is  best  in  a  case  like 
this.  This  consists  of  an  electric  vibrating  bell,  or  a  lamp, 
mounted  in  a  case  with  a  number  of  dry  cells,  and  forms  a 
convenient  test  box,  but  is  not  so  efficient  as  the  magneto,  as 


FIG.  236. — Magneto  and  bell  test.        FIG.  237. — Simple  fuse-testing  panel. 


FIG.  238. — Testing  set  consisting  of  several  dry  cells  mounted  in  a  portable  case 
with  a  bell  and  small  lamp. 

the  voltage  is  low  and  the  test  current  comparatively  high.  It 
will  not  show  up  many  high-resistance  grounds  which  might 
be  detected  by  the  magneto.  The  magneto  develops  a  com- 
paratively high  voltage  and  takes  very  little  bell-operating 
current. 


254  PROTECTIVE  RELAYS 

If  low-potential  alternating  current  is  available,  a  small 
potential  transformer,  properly  equipped  with  protective  lamps 
and  test  leads,  forms  an  excellent  piece  of  apparatus  for  testing 
for  grounds,  opens,  and  shorts.  In  this,  the  primary  is  con- 
nected in  series  with  a  fuse  to  a  110-v.  A.C.  circuit.  The 
secondary  should  have  taps  connected  to  a  dial  switch  so  that 
any  voltage  from  100  to  2,000  may  be  obtained  in  100-v. 
steps. 

A  galvanometer  or  a  low-reading  voltmeter  may  be  used 
in  series  with  a  few  dry  cells  for  the  same  purpose.  This  has 
the  advantage  that  the  resistance  of  the  ground,  open  or  short 
may  be  roughly  calculated  from  its  indication. 

Testing  for  Opens,  Shorts  or  Grounds. — Let  us  assume  a 
factory  running  a  number  of  motors  and  lighting  circuits;  say 
one  department  reports  that  a  certain  motor  will  not  run.  The 
repairman  takes  his  voltmeter  and  magneto  set,  and  going 
to  the  department  where  the  motor  stopped  should  first  con- 
duct an  investigation  among  the  ones  who  operate  the  motor, 
and  diagnose  the  case  much  in  the  same  way  as  a  physician 
diagnoses  human  ills.  Was  there  any  unusually  heavy  load 
applied  when  the  motor  stopped?  Did  the  motor  heat  up 
excessively?  Did  someone  throw  the  starting  compensator 
handle  from  starting  to  running  position  too  quickly? 

If  the  trouble  is  merely  a  blown  fuse,  the  cause  of  blowing 
should  be  determined  to  avoid  recurrence.  If  no  one  can  report 
any  difficulty,  remove  the  fuses  and  test  the  voltage  on  the  line 
side  of  the  fuse.  Then  test  the  fuses  separately  with  the  magneto, 
or  with  a  special  fuse-testing  panel,  such  as  shown  in  Fig.  237. 
To  do  this,  connect  the  two  fuse  terminals  to  the  magneto  out- 
fit, and  turn  the  handle.  If  the  bell  rings,  the  fuse  is  intact, 
but  if  it  does  not  ring,  then  it  is  blown  out.  On  a  two-wire 
circuit,  the  voltage  may  be  tested  on  the  load  side  without 
removing  the  fuses,  where  a  lack  of  voltage  will  indicate  a  blown 
fuse.  But  this  cannot  be  done  on  a  three-wire  or  a  polyphase 
circuit  unless  it  be  definitely  ascertained  that  there  is  no  con- 
nected load,  as  the  current  from  another  phase  may  feed  back- 
ward through  the  load  and  operate  the  voltmeter,  thus  giving 
an  indication  of  good  fuses.  It  takes  at  least  two  blown  fuses 
to  kill  the  secondary  voltage  of  a  polyphase  circuit  with  a  con- 


LOCATING  FAULTS  IN  FEEDERS  AND  WIRING         255 


nected  load.     Figure  239  shows   the  proper  method  to  pursue 
in  locating  a  blown  fuse. 

As  we  are  dealing  with  feeder  and  wiring  faults,  assume  that 
the  fuses  are  good  but  there  is  no  voltage;  or  on  a  polyphase 
circuit,  that  there  is  no  voltage  on  at  least  one  phase.  Proceed 
to  the  next  fuse  junction  nearest  to  the  source  and  perform 
the  same  test.  Perhaps  a  blown  fuse  will  be  found  here  that 
kills  the  voltage  at  the  motor-fuse  block.  Assume  for  the  first 
case  that  a  blown  fuse  is  found  and  upon  replacement  it  blows 
out  immediately.  Since  the  other  end  of  the  line  is  open- 
circuited,  it  shows  that  there  is  a  short  or  a  ground  in  this 


Line 


L    e 


Load 


FIG.  239. — At  left,  when  load  is  disconnected,  fuses  are  tested  by  connecting 
voltmeter  between  the  line  side  of  one  fuse  and  the  load  side  of  another.  At 
right,  testing  around  the  fuse. 

line.  Now  remove  all  fuses  or  connections  on  both  ends  of  the 
line.  If  the  line  is  in  metal  conduit,  connect  one  magneto 
lead  to  the  metal  conduit,  or  if  it  is  open  wiring,  connect  the 
lead  to  the  nearest  water  pipe.  Connect  the  other  magneto  lead 
to  one  of  the  wires  in  the  isolated  section.  Turn  the  magneto 
briskly,  and  if  the  bell  rings,  the  wire  is  grounded;  no  ring 
indicates  an  ungrounded  line.  Try  the  other  lines  in  succession. 
Assume  for  the  present  that  all  lines  in  the  section  under  test 
are  free  from  grounds. 

Now  connect  the  two  magneto  leads  one  to  each  of  two  wires 
in  the  section  under  test  and  turn  the  handle.  A  ringing  bell 
indicates  a  short-circuit  between  two  wires,  which  in  this  case 
would  be  the  cause  of  the  blown  fuses. 

It  is  very  seldom  that  a  feeder  or  heavy  wire  will  open-circuit, 
but  the  test  is  made  by  grounding  the  far  ends  of  the  wires  in  the 


256  PROTECTIVE  RELAYS 

section  under  test  and  then  testing  with  the  magneto  between 
each  at  the  opposite  end.  A  ring  will  indicate  a  continuous 
circuit,  while  no  ring  will  indicate  an  open  circuit. 

Open  circuits  frequently  occur  in  lamp  cord  and  light  wiring 
by  the  wire  breaking  inside  the  insulation,  while  on  the  outside 
it  appears  to  be  intact.  These  cases,  however,  are  not  difficult 
to  find  as  they  generally  occur  in  single  units  such  as  lamps 
and  small  motors,  and  seldom  on  distributing  feeders  which  carry 
a  heavy  load. 

Accurately  Locating  the  Short-circuit. — After  localizing  the 
fault  to  a  section  between  two  fuse  centers,  the  fault  often 
may  be  accurately  located  by  a  careful  inspection,  especially 
in  open  wiring,  but  if  it  is  in  conduit,  and  quite  a  long  run  in  the 
section  under  test,  some  method  is  necessary  to  determine  the 
approximate  location  so  that  only  the  wires  affected  in  smaller 
section  between  two  outlets  need  be  pulled  out. 

A  short-circuit  may  be  most  easily  localized  by  a  direct- 
current  reversing  commutator  and  a  compass.  Or  a  simple  rever- 
sing switch  operated  by  an  assistant  has  the  same  result.  First 
make  sure  that  the  line  is  dead  at  both  ends;  then  connect  a 
6-v.  storage  battery,  through  an  adjustable  resistor,  ammeter 
and  double-pole  double-throw  reversing  switch,  to  the  faulty 
line.  Adjust  the  resistance  so  that  5  or  10  amp.  flows  through 
the  short.  The  assistant  is  instructed  to  reverse  the  switch 
about  every  10  sec.  The  cover  is  removed  from  the  nearest 
junction  box,  and  the  compass  held  against  the  wire.  If  the 
fault  is  past  this  box,  the  compass  will  reverse  every  time 
the  assistant  reverses  his  switch.  If  the  compass  reverses, 
proceed  to  the  next  box  and  see  if  the  compass  works.  If  so 
it  shows  the  fault  to  be  further  on,  so  a  test  is  made  at  the  next 
box,  and  so  on  until  a  junction  box  is  reached  where  the  compass 
does  not  deflect. 

A  similar  method  that  may  be  used  where  alternating  current 
is  available  is  formed  by  passing  5  or  10  amp.  through  the  faulty 
cable  and  the  short-circuit.  Instead  of  the  compass,  a  small 
piece  of  transformer  iron,  about  1  in.  wide  and  several  inches 
long,  is  bent  in  a  semicircle  and  about  10  turns  of  No.  18  or 
20  B.  &  S.  gage  insulated  magnet  wire  wound  around  the 
center.  Flexible  leads  are  used  to  connect  this  winding  to  a 


LOCATING  FAULTS  IN  FEEDERS  AND  WIRING         257 

telephone  receiver.     Figure  240  gives  an  illustration  showing 
the  use  of  both  methods. 

The  junction  box  nearest  the  application  of  current  is  opened 
and  the  soft  iron  held  over  the  wire.  A  buzz  indicates  that  the 
fault  is  further  on;  so  each  outlet  is  tested  in  turn  until  one 
is  reached  where  no  buzz  is  heard,  thus  indicating  that  the  fault 
lies  between  this  outlet  and  the  one  last  tested.  The  remedy 
lies  in  pulling  out  the  faulty  cable,  or  calculating  the  exact  loca- 
tion of  the  fault  and  cutting  the  conduit  at  the  fault  so  that  an 
outlet  and  splice  may  be  made  without  interfering  with  the  rest 


FIG.  240. — Two  methods  of  locating  short-circuits  in  a  cable.  With  direct 
current  a  compass  is  used,  with  alternating  current  the  tester  uses  telephone 
receivers  and  the  detector  coil  shown  encircled. 


of  the  cable.  While  this  method  is  described  only  for  small 
systems,  yet  it  is  to  be  understood  that  the  same  principles  only 
on  a  larger  scale  may  be  applied  to  long-distance  transmission 
lines. 

Localizing  a  Ground. — The  methods  used  in  locating  a  ground 
are  very  similar  to  those  used  for  shorts.  Instead  of  passing 
the  current  through  the  two  wires  and  the  short,  the  source  is 
connected  between  the  grounded  wire  and  any  convenient  ground, 
the  resulting  current  passing  through  the  accidental  ground 
on  the  wire.  Either  direct  current  and  compass  may  be  used, 
or  alternating  current  and  telephone  receiver. 

This  method  is  very  effective  with  radial  systems  as  it  is 
only  necessary  to  test  each  pair  of  feeders  at  the  branches  and 

17 


258  PROTECTIVE  RELAYS 

follow  out  the  wires  that  indicate  grounds  or  shorts  until  the 
fault  is  found. 

Calculating  the  Location  of  Short  or  Ground. — If  the  short 
or  ground  is  on  a  single  line  which  may  be  isolated  by  discon- 
necting at  both  ends,  and  if  the  resistance  is  accurately  known, 
then  the  location  of  a  fault  may  be  quite  accurately  found 
by  a  measurement  of  the  resistance  either  by  a  Wheatstone's 
bridge  or  by  the  volt  ammeter  method. 

For  instance,  assume  two  wires  500  ft.  long  and  each  wire 
measuring  0.05  ohm. 

Passing  10  amp.  (adjusted  by  rheostat  and  measured  by 
an  accurate  ammeter)  through  the  short,  assume  the  potential 
drop  across  the  cable  to  be  0.8  v.  or  800  m.v.  The  resistance 
of  the  two  sections  of  wire  as  far  as  the  short,  plus  the  resistance 
of  the  short,  is  0.08  ohm.  Now  connect  the  two  far  ends  of 
the  cable  tightly  together,  and  measure  again.  Assume  the 
voltmeter  now  shows  0.733  v.  or  773  m.v.,  thus  indicating  that 
paralleling  the  resistance  of  the  short  with  the  remaining  section 
of  good  cable  reduces  the  resistance  from  0.08  to  0.0733  ohm. 
Consequently  the  resistance  of  the  good  cable,  as  far  as  the 
short,  may  be  calculated  from  the  formula: 

X  =  C  -  V(b-c)(a-c) 

in  which  X  =  the  total  resistance  of  cable  to  short. 

a  =  the  total  known  resistance  of  good  wires. 
b  =  resistance  measured  with  ends  open. 
c  =  resistance  measured  with  ends  closed. 

Or  in  our  example: 

X  =  0.0733  -  V(0.08  -  0.0733)  (0.1  -  0.0733) 

which  equals  0.06  ohm. 

This  is  the  resistance  of  two  wires  to  the  short,  so  each  wire 
will  be  0.03  ohm.  If  the  resistance  of  500  ft.  is  0.05  ohm, 
then  0.03  ohm  will  represent  300  ft.,  showing  that  the  short 
is  300  ft.  from  the  beginning  of  the  cable. 

A  ground  may  be  located  in  the  same  manner,  if  the  resistance 
of  the  cable  be  known,  by  measuring  the  resistance  between  cable 
and  ground  with  the  far  end  of  cable  insulated  and  then  measur- 
ing the  resistance  with  the  far  end  grounded,  although  in  this 


LOCATING  FAULTS  IN  FEEDERS  AND  WIRING         259 

case  accuracy  is  not  high  unless  the  line  be  comparatively  long 
and  high  resistance  and  the  ground  resistance  practically  zero. 
Two-ammeter  Method. — Another  way  of  approximately 
locating  a  ground  in  a  heavy  feeder  is  by  the  two-ammeter 
method,  connecting  an  ammeter  in  each  leg  of  the  circuit  and 
noting  the  division  of  the  current.  To  do  this  first  clear  both 
ends  of  the  line  and  then  join  the  two  far  ends  together  tightly. 
Then  connect  a  5-amp.  D.C.  ammeter  in  series  with  each  wire, 
connecting  the  opposite  side  of  the  ammeters  together;  from 
this  joint  connect  a  storage  battery  and  resistor  to  the  ground. 
The  connections  are  shown  in  diagram  in  Fig.  241. 

Ends  connected  together 
Cable  under  Test 

Ammeters  /  Q  1 

/       JZ—L j^        cr 


Accidental  Ground 
gk- Storage  Battery 


FIG.  241. — Locating  ground  on  a  heavy  feeder  by  measuring  the  division  of 
current  by  ammeters  A  and  B. 


The  current  through  meter  A  needs  to  go  only  through  part 
of  one  feeder  to  reach  the  break,  but  the  current  through  meter 
B  must  not  only  traverse  the  total  length  of  one  cable,  but  also 
the  remaining  section  of  the  grounded  cable.  Since  the  currents 
divide  inversely  proportional  to  the  resistance  of  the  cable 
they  must  travel  and  since  this  resistance  is  proportional  to  the 
length,  it  follows  that  the  ratio  of  the  two  meters  is  inversely 
proportional  to  the  two  lengths  of  cable.  Or  the  ratio  of  one 
reading  to  the  sum  of  the  two  readings  is  the  same  as  the  ratio 
of  the  opposite  section  to  the  break  is  to  the  total  length. 

For  instance,  if  meter  A  reads  3  amp.  and  meter  B  reads  2  amp. 
and  since  this  division  is  inversely  proportional  to  the  resistances, 
the  proportion  is  3  :  2  =  AG  :  BG.  Or  call  the  whole  circuit  100 
per  cent,  then  3:2=  (100-AG)  :AGor2:5=X:  100.  Solving 
this  for  BG  or  X  gives  40,  which  means  that  the  fault  is  40  per 
cent  of  the  total  length  of  the  cable  away  from  the  meters. 


260  PROTECTIVE  RELAYS 

For  very  accurate  results  with  the  two-ammeter  method, 
it  is  necessary  to  figure  the  resistance  of  the  ammeters  and  con- 
necting cables  as  so  many  feet  of  equivalent  cable.  For  instance, 
if  the  cable  being  tested  is  500  ft.  long  (1,000  ft.  for  two  con- 
ductors) and  has  a  resistance  of  0.05  ohm  per  500  ft.,  and  if 
the  resistance  of  the  meter  and  connections  from  the  dividing 
point  to  the  cable  is  found  to  be  0.002  ohm,  then  it  is  evident 
that  the  meter  and  connections  have  a  resistance  equivalent 
to  20  ft.  of  cable.  To  measure  the  resistance  of  the  meters, 
pass  about  5  amp.  through  the  meter  on  all  connections, 
and  then  with  a  millivoltmeter  take  the  drop  from  the  dividing 
point  between  the  meters  to  the  nearest  point  on  the  cable, 
including  all  connections  and  joints.  Say  this  is  100  m.v. 

E 
Then  the  resistance  is  R  =-v  or  0.1  v.  -f-  5  amp.  =  0.002  ohm. 

Since  the  cable  is  0.05  X  500  ft.  or  0.0001  ohm  per  ft.,  it  fol- 
lows that  it  takes  20  ft.  to  make  up  a  resistance  of  0.002  ohm. 
If  the  other  meter  measures  the  same,  then  in  making  the  final 
calculation,  it  is  necessary  to  figure  the  total  length  of  cable 
as  (500  +  500  +  20  +  20)  or  1,040  ft.  instead  of  merely  1,000 
ft.  So  in  our  previous  example  the  fault  would  figure  40  per 
cent  of  1,040  or  416  ft.  away  from  the  end  being  tested.  But 
we  already  know  that  the  meter  is  equivalent  to  20  ft.,  so  we  must 
subtract  this,  making  the  actual  location  of  the  fault  416  —  20  or 
396  ft.  away. 

The  Fault  Localizer. — A  much  safer  method  to  locate  a  ground 
in  a  heavy  cable  or  feeder  is  by  the  Westinghouse  fault  local- 
izer.  This  instrument,  a  diagram  of  which  is  shown  in  Fig.  242, 
has  three  terminals  two  of  which  are  connected  to  the  respective 
wires  in  a  two-wire  feeder  and  the  other  through  a  source  of 
D.C.  potential,  ammeter  and  lamp  bank  to  the  ground.  The 
central  knob  is  then  turned  until  the  galvanometer  shows 
no  deflection  upon  pressure  of  the  key.  The  point  on  the  scale 
that  now  lies  directly  below  the  0  mark  indicates  directly  the 
percentage  of  cable  length  to  the  ground.  One  post  is  marked 
"red"  and  one  "black,"  so  if  the  reading  appears  in  red  on  the 
scale,  then  the  ground  is  on  the  line  connected  to  the  "red" 
post.  For  example  if  the  scale  showed  60  per  cent  in  black, 
then  it  would  be  known  that  the  line  connected  to  the  black 


LOCATING  FAULTS  IN  FEEDERS  AND  WIRING         261 

post  was  grounded,  and  if  the  total  length  of  line  was  100  ft., 
then  the  ground  would  be  60  ft.  (60  per  cent  of  100)  from  the 
instrument. 

In  principle  of  operation,  the  fault  localizer  is  somewhat 
similar  to  a  specially  constructed  Wheatstone  bridge.  As  will 
be  seen  in  the  diagram,  Fig.  242,  there  is  a  heavy  shunt  with 
three  terminals  which  carries  the  main  current,  thus  allowing 
a  comparatively  small  slide  wire  to  be  used.  The  slide  wire 
is  wound  on  a  circular  spool  in  two  sections,  and  on  each  section 


Cable  under  Test 
M  Cahle  Jumper, 


Galvanometer 


^ 

Accidental 
Ground. 


FIG.  242. — Diagram  of  connections  of  Westinghouse  fault  localizer. 

is  a  movable  contact.  It  will  be  readily  seen  that  the  current 
between  the  center  post  and  the  fault  divides  through  the  two 
sides  of  the  shunt  inversely  proportional  to  the  resistance  of  the 
loop  from  the  instrument  to  the  fault.  The  currents  in  the  slide 
wires  being  proportional  to  the  current  in  the  shunts,  it  fol- 
lows that  by  moving  the  contacts  so  they  span  more  of  one  and 
less  of  the  other  wire,  there  will  be  a  point  reached  where  a 
balance  is  reached.  This  point  indicated  on  the  movable  scale 
gives  the  location  of  the  fault  in  percentage  of  the  total  loop. 

L.  &  N.  Power  Bridge. — When  an  attempt  is  made  to  local- 
ize a  fault  in  a  very  heavy  and  comparatively  short  cable,  it  will 
be  found  that  the  utmost  care  must  be  taken  to  avoid  contact 
resistance  in  the  joints  between  the  instrument  and  the  cable. 
Even  the  slight  resistance  of  the  connecting  leads  may  introduce 
a  serious  error.  For  instance,  if  the  contact  resistance  is  0.001 
ohm  and  the  wires  were  of  such  a  size  that  0.001  ohm  were 
equal  to  the  resistance  of  20  ft.  of  the  cable,  then  it  means 
that  there  would  be  an  error  of  20  ft.  in  the  location  of  the 
fault.  This  clearly  demonstrates  that  the  necessity  of  making 
as  perfect  a  contact  as  possible  between  the  instrument  and 
cable  cannot  be  too  strongly  emphasized. 


262 


PROTECTIVE  RELAYS 


To  partly  overcome  this  difficulty,  the  Leeds  &  Northrup 
power  bridge  has  heavy  leads  permanently  connected  to  the 
bridge  and  heavy  clamps  on  the  ends  for  securely  clamping  to 
the  cable.  This  is  shown  in  Fig.  243. 

In  principle  of  operation,  this  bridge  consists  of  a  slide-wire 
bridge  with  a  very  sensitive  galvanometer  mounted  in  the  carry- 
ing case.  The  low  resistance  slide  wire  is  mounted  on  a  circular 


FIG.  243. — Leeds  &  Northrup  power  bridge. 


block  inside  the  case  and  arranged  with  a  very  positive  movable 
contact.  This  contact  is  rigidly  attached  to  a  shaft  which 
carries  a  knob  and  pointer  moving  over  a  calibrated  scale.  The 
wire  is  made  large  enough  to  carry  about  5  amp.,  thus  giving 
a  readable  deflection  for  a  short  movement  of  the  contact.  If 
the  occasion  demands,  this  current  may  be  increased  to  8  amp. 
to  obtain  very  accurate  results,  but  this  heavy  current  should 
not  be  left  on  longer  than  is  absolutely  necessary. 

The  scale  is  divided  into  1,000  divisions,  but  the  leads  are 
arranged  to  equal  10  divisions  of  the  slide  wire,  so  the  pointer 
will  only  go  from  10  to  990  on  the  scale.  It  will  thus  be  seen  that 


LOCATING  FAULTS  IN  FEEDERS  AND  WIRING 


263 


the  slide  wire  actually  begins  at  the  ends  of  the  cable,  thus  entirely 
eliminating  lead  resistance. 

In  using  the  instrument  to  locate  grounds,  the  line  is  first 
cleared  at  both  ends  and  then  one  end  of  the  cable  is  tightly 
clamped  together.  Care  must  be  taken  to  avoid  contact  resis- 
tance here. 

The  battery  is  connected  to  posts  marked  Ba.  The  post  marked 
Gr  must  be  securely  grounded.  Sufficient  battery  must  be 
used  to  obtain  a  readable  deflection  from  a  slight  change,  or 
resistance  must  be  inserted  if  the  current  is  too  large.  The 
connections  are  shown  in  Fig.  244. 


FIG.  244.  —  Diagram  of  connections  of  L.  &  N.  power  bridge. 

The  bridge  locates  the  fault  by  the  Murray  loop  method. 
If,  for  instance,  the  bridge  is  balanced  so  that  the  galvanometer 
shows  no  deflection  when  the  pointer  is  at  300  and  the  key  is 
pressed,  then  it  means  that  the  distance  from  block  A  to  the 


fault  is 


1,000 


of  the  total  distance. 


LOCATION 


OF    FAULTS    WHEN    THE    LOOP    IS    COMPOSED    OF 
DIFFERENT  CROSS-SECTION  CABLES 


When  there  are  cables  of  varying  cross-section  in  the  loop 
being  tested,  the  fault  is  usually  located  by  reducing  the  cables 
to  equivalent  lengths  of  one  size  cable. 

For  instance,  in  diagram,  Fig.  244,  assume  that  there  are 
three  sections  of  cable  of  various  sizes  and  that  these  sections 
are  as  follows:  Length  A  to  E  is  composed  of  550  yd.  of  25,000 
cir.  mils,  length  EF  is  500  yd.  of  40,000  cir.  mils,  and  length 
FC  is  1,050  yd.  of  30,000  cir.  mils.  These  lengths  must  be 
reduced  by  calculation  to  equivalent  lengths  of  one  size,  and  for 
this  purpose  it  is  best  to  select  the  largest  size.  Since  the 


264  PROTECTIVE  RELAYS 

resistance  is  inversely  proportional  to  the  cross-section  and 
directly  proportional  to  the  length,  it  follows  that  the  calculation 
is  merely  an  inverse  proportion.  To  reduce  the  first  length, 
the  equation  becomes: 

550  :  25,000  =   x:  40,000 

which  gives  x  as  880  yd.,  meaning  that  880  yd.  of  40,000-cir. 
mil  cable  is  equal  in  resistance  to  550  yd.  of  25,000-cir.  mil  cable. 
Reducing  the  rest  in  a  similar  manner  gives  the  following: 

550  yd.  of  25,000  cir.  mils  =  880  yd.  of  40,000  cir.  mil 
500  yd.  of  40,000  cir.  mils  =  500  yd.  of  40,000  cir.  mils. 
1,050  yd.  of  30,000  cir.  mils  =  1,400  yd.  of  40,000  cir.  mils. 

This  makes  the  total  resistance  of  the  loop  equivalent  to 
2,780  yd.  of  40,000.  Now  if  the  bridge  balances  at  a  reading 
of  372.5  this  indicates  that  the  fault  is  37.25  per  cent  of  the  total 
distance  or  1,035.5  equivalent  yards  from  E.  Of  this,  880 
are  in  the  stretch  AE,  leaving  only  1,035.5  —  880  or  155.5  yd., 
which  is  the  distance  from  E  to  the  fault. 

Burning  Out  the  Fault. — When  the  fault,  either  cross,  ground 
or  partial  open,  is  of  a  high  resistance  and  it  is  impossible  to  locate 
it  by  ordinary  methods,  it  is  sometimes  permissible  to  burn 
it  out.  This,  however,  must  be  attempted  only  with  the  greatest 
precautions  such  as  having  pails  of  sand  or  reliable  fire  extin- 
guishers ready  for  immediate  use  should  the  burning-out  process 
start  a  fire. 

To  do  this,  connect  a  high  potential  so  that  it  feeds  current 
through  the  fault  and  then  increase  the  current  until  some- 
thing happens.  This  something  may  be  a  fire,  a  melted  conduit, 
a  ruined  section  of  wire,  additional  trouble  communicated  to 
other  sections  of  wire,  or  merely  a  carbonizing  of  the  fault. 
If  the  fault  becomes  carbonized  sufficiently  to  pass  about 
5  amp.  through  from  a  low-voltage  battery,  then  it  is  easy  to 
locate  the  fault  by  the  previously  described  methods. 

In  any  case  burning  out  a  fault  is  a  method  which  must  be 
resorted  to  only  in  an  extreme  case  as  practically  all  faults 
may  be  located  in  a  much  safer  and  gentler  manner  by  a  little 
clear  thinking  and  sound  reasoning. 


INDEX 


Accuracy,  curve,  229 

of  potential  transformer,  124 
Actual  testing,  239 
Adjustable  slide  resistor,  235 
Adjustments,  testing,  242 
time,  27,  36,  78 
tripping  current  in  G.  E.,  78 
Advantages  of  Z-connection,  120 
Air-valves,  G.  E.  early  type,  37 

modern  type,  36 
Alternating  current  disturbances,  98 

relays,  testing,  231 
also  see  Principles  of  operation. 
Alternator  constants,  105 
American  Institute  nomenclature,  4 
Ammeter,  Westinghouse  induction, 

236 

Ampere-turns     in     current     trans- 
former, 116 
Apparatus    required    for    locating 

faults,  252 
for  testing,  223,  232 
Application   of   A.C.   power   direc- 
tional relays,  164,  183 
D.C.     power     directional     re- 
lays, 55 

overload  relays,  132-152 
also  see  Protection. 
Attachments   for    circuit   breakers, 

see  Releases. 

Automatic  oil  switches,  7 
Auxiliary  relays,  17 
bell  ringing,  198 
contactor  switches,  71,  90 
interlocking,  96 
multi  contact,  207 
nomenclature,  6 


Auxiliary  relay  switches,  205 
transfer,  19,  208 


B 


Balanced  protection,  generators,  148 

parallel  feeders,  155,  173 
Banks,  transformer  protection,  143 
Batteries,  storage,  protection  of,  55 

testing,  225 

Battery  and  bell  test,  253 
Beck  Bros,  carbon  rheostat,  234 
Bellows  type  relays,  General  Electric 
unit  plunger  type,  28,  35, 
39 

objections  to,  38 
quick  resetting  valves,  36, 

37 

testing  for  grounds,  222 
vs.  induction,  82 
Westinghouse  overload,  31 
Bell  ringing  relays,  198 
Burning  out  the  fault,  264 


Cables,   resistance  and  impedance, 

103 
Calculation    location    of    short    or 

ground,  258 

short  circuit  current,  101 
Carbon  circuit  breaker,  7 
Carbon  rheostat,  225,  234 
Characteristics  of  A.C.  disturbances, 

98 

relays,  108 

time  of  circuit  breakers,  180 
Circuit  breakers,  7 

attachments,  see  Releases. 


265 


266 


INDEX 


Circuit-breakers,          diagrammatic 

scheme,  9 

releases,  see  Releases, 
timing,  216 
typical   time   characteristics, 

180 
Circuit-closing  relays,  diagrams,  17, 

34,41 

G.  E.  inverse  overload  bel- 
lows, 39 

Westinghouse  overload  bel- 
lows, 29,  31 

Circuit-opening  relays,  18 
diagrams,  40,  42 
G.  E.  dashpot  inverse  over- 
load, 38 

Clips  for  testing,  226 
Compass  test,  276 

Compensation    in     current     trans- 
former, 113 

in  potential  transformer,  124 
for  temperature  in  G.  E.  relay,  81 
Compensators — torque,  69,  92 
Westinghouse  diagram,  70 
Condit,  relays,  24 

overload  horizontal  bus,  25 
vertical  bus,  25 

Constants,    alternator    and    trans- 
former, 105 

Contactor  switch,  71,  90 
diagram  of  modern,  72 

of  old,  72 
,    exploded  view,  73 

location  in  relay,  73 
Contacts,  71 
Continuity  indicator,  74 
Cross  connected  power  directional 

relays,  165,  169 
systems,  disadvantage  of,  171 
Current,  calculation  of  short  circuit, 

101 

relays,  66 
nomenclature,  5 
tap  plates,  G.  E.  induction  over- 
load, 78 

Westinghouse  induction  over- 
load, 69 


Current  transformer,  ampere  turns, 

116 

effects  of  secondary  load,  115 
hole  type,  237 
inherent  errors,  113 
magnetization  of  core,  114 
necessity  for  3  on  3  phase,  119 
opening  of  secondary,  122 
ratio  errors,  113 
single-phase  groupings,  117 
temperature  rise,  116 
three-phase  groupings,  123 
three-wire,  117 
through  type,  237 
two-phase  groupings,  117, 118 
Westinghouse,  237 
wrong  connections,  119 
Curves,  current  decrease  on  genera- 
tor short,  100 

ratio    error   in    current    trans- 
formers, 114 
relay  accuracy,  229 
results,  246 
tables  and,  228 
time  load,  77 
of  fuse,  20 
of  G.  E.  bellows,  20 
of  G.  E.  definite  time,  21 
of  G.  E.  induction  overload, 

22, 180 
of  Westinghouse  bellows,  33, 

178 
of  induction  overload,  22, 

179 
Cycle  counter,  212,  240 


D 


D'Arsonval  type  relay,  power  direc- 
tional, 46 

principles  of  operation,  45 
temperature,  201 
testing  for  grounds,  222 
Dashpot   type   circuit   opening   in- 
verse overload,  38 
Definite  time  delays,  6,  21,  26 
relays,  29 


INDEX 


267 


Delta,  connections,  120 

and  Z-connection  table,  121 
Determining  phase  rotation,  131 
Disadvantage  of  cross-connected 

system,  171 

Diagrammatic    schemes,    see   Sche- 
matic diagrams. 

Diagrams,  battery  protection,  56 
circuit  closing  separate  trip,  17 
single-phase,  34 
standard   General   Electric, 

41 

Westinghouse,  34 
opening,  18,  40 

standard  General  Electric, 

42 

Westinghouse,  41 
Continuity  indicator,  74 
D.C.  ring  system,  63 
differential    generator    protec- 
tion, 149 

double    contact    power    direc- 
tional, 170,  172 
elementary  overload  protection, 

133 

parallel  feeders,  61,  85 
radial  system,  30,  175,  177 
ring  system,  184 
failure  of  prime  movers,  59,  60 
fuse  testing,  255 
G.  E.  for  power  directional,  167, 

168 

internal,  94 
insulation  testing,  222 
modern  contactor  switch,  72 
network  protection,  187 
old  contactor  switch,  72 
overvoltage  signal,  65 
parallel   feeder   radial   system, 

181 

power  directional  relays,  165,166 
protection   of   battery  and  ro- 
tary, 58 
of  parallel  feeders,  154,  155, 

157,  161,  163,  173 
of  transformer,   145,  also  see 
Protection. 


Diagrams,  radial  network,  176,  182 
series  trip,  40 

service  restoring  relays,  18,  195 
split  conductor  system,  159 
standard  G.  E.  for  S.  P.  induc- 
tion overload,  136 
for  three-phase  four  wire, 

140 

grounded,  139 
ungrounded,  138 
for  two-phase,  137 
Westinghouse,  for  induction 

overload,  135 
for  power  directional,  166 
watt  relays,  129 
temperature  relays,  192,  204 
testing,  voltage,  relays,  248 
with  cycle  counter,  214,  239 
without  disconnecting,  241 
three-phase  overload,  134 
three-wire  current  transformer, 

117 
timing    protection    equipment, 

216 

transfer  relay,  209 
two-current  transformer  on  3- 

wire,  117 

two-phase  overload,  134 
typical  layout,  218 
Westinghouse  bellows,  34,  41 
cross-connected,  169 
induction  overload,  135 
overload  telegraph,  192 
power  directional,  170 
reverse  phase,  194 
torque  compensator,  70 
Z-connection,  122 
Differential  power  directional,   96, 

170 

nomenclature,  6 
protection  parallel  feeders,  156 
Direct  current  power  directional  re- 
lays, 44 

applications,  55 
relays,  testing,  220 
ring  systems,  62 
Directional  relay,  nomenclature,  5 


268 


INDEX 


Disturbances,    characteristics  of 

A.C.,  98 
Double  contact  relays,  172 


E 


Effect  of  frequency  on  transformer, 

124 

of  low  voltage,  105 
of  overload,  98 
of  secondary  load,  115 
of  stray  fields,  92 
of  unbalanced  short  circuits,  106 
of  wave  form,  124 

Electrically  operated  circuit  break- 
ers, 7 

Elementary  diagrams,  see  Diagrams 
and  also  Schematic  dia- 
grams. 

Errors,  inherent  in  current  trans- 
former, 113 


Failure  of  prime  movers,  59 
of  rotary  converter,  58 

Fault  localizer,  260 

Faults,  burning  out,  264 
location  of,  251 

Feeders,  see  Parallel  feeders. 

Fire  risk  from  fuses,  1 

First  fuse,  1 

Flexible  lamp  load,  233 

Frequency  relay,  nomenclature,  5 

Fully  automatic  circuit  breaker,  7 

Fuses,  fire  risk  from,  1 
testing,  253,  255 
time-load  curve  of,  20 


G 


General    Electric    Relays,    bellows 

overload,  39 
dashpot  type,  38 
induction  overload,  75 
power  directional  D.C.,  48 

A.C.,  93 
relay  switch,  206 


General   Electric    Relays,    solenoid 

voltage,  190 

strap  wound  overload,  27 
unit  type,  28,  35 

Generators,  protection  of,  132,  148 
Grounds,  calculation  of  location,  258 
protection  against,  150 
testing  for,  221,  254 
Groupings,  transformer  current,  sin- 
gle-phase, 117 
two-phase,  117 
three-phase,  123 
potential,  two-phase,  125 
three-phase,  126 


H 


Heavy  current  relay  testing,  241 
High-tension  relays,  210 
Hole-type  transformer,  113 
Horizontal  bus  type  relays,  Condi t 

overload,  25 

G.  E.  power  directional,  49 
How  time  delays  are  obtained,  23 


Impedance,  overhead  lines,  102 
three  conductor  cable,  103 
Index  plate,  G.  E.  induction  relay, 

77 

operator's  transcript,  78 
Westinghouse   induction  relay, 

69 

Indicator,  continuity,  74 
Inductance,  overhead  lines,  102 
Induction  ammeter,  237 
Induction  type  relays,  66 

G.  E.  overload,  75 

power  directional,  93 
vs.  solenoid-plunger,  82 
Westinghouse  overload,  65 
power  directional,  90 
reverse  phase,  193 
temperature-load,  203 
voltage,  189 
Industrial  plants,  protection  of,  110 


INDEX 


269 


Inherent    errors    in    current    trans- 
formers, 113 

Installing  D.C.  relays,  220 
Instantaneous  time,   qualifying 

terms,  6 
Instrument  transformers,  112  also  see 

Transformers. 
Insulated  fuses,  1 
Insulation  test,  221 
Interlocking  relays,  96 
Internal    diagrams,    G.    E.    power 

directional,  94 

Westinghouse    power    direc- 
tional, 91 

Inverse  definite  time  delays,  21 
time  discrimination,  153 
limit,  20 

qualifying  terms,  6 
Inverted  series  transformer,  242 


Lamp  load,  flexible,  233 
Latest  developments,  power  direc- 
tional relays,  89 
Layout,  typical,  217 
Leeds  and  Northrup  power  bridge, 

262 
Lines,  overhead,  resistance,  etc.,  102 

protection  of,  132,  150 
Load,  boxes,  224 

effect  of  secondary,  115 

flexible  lamp,  233 

on  transformers,  83 

phantom,  235 

series  type  relays,  223 

testing,  223,  232 

transformer,  127 
Localizing  grounds,  257 
Locating  faults,  251 

short  circuits.  256 
Location  of  contactor  switch,  73 
Locking  relay,  nomenclature,  6 
Low  current  relays,  109 

use  in  pilot  wire  systems, 

163 
Low  voltage,  effect  of,  105 


M 


Magnetization  of  core,  114 
Magneto  and  bell  test,  253 
Making  Z-connections,  122 
Miscellaneous  relays,  189 
Motors,  protection  of,  132 
Mueller  Electric  Co.,  testing  clip, 

226 
Multi-contact  relay,  207 


N 


Nature  of  short  circuits,  100 
Necessity  for  three  transformers,  119 
Networks,  182 

protection  of,  185 
Nomenclature,  4 

auxiliary  relay,  6 

current  relay,  5 

differential  relay,  6 

directional  relay,  5 

frequency  relay,  5 

locking  relay,  6 

open-phase  relay,  6 

phase  rotation  relay,  5 

polarity  directional  relay,  5 

power  directional  relay,  5 

protective  relay,  5 

qualifying  terms,  6' 

signal  relay,  6 

temperature  relay,  5 

trip  free  relay,  6 

voltage  relay,  5 

watt  relay,  5 


Objections  to  bellows  type,  38,  82 

to  fuses,  1 

to  series  trip,  18 
Obtaining  time  delays,  23 
Oil  circuit-breakers,  7 
Oil-damped  relays,  39 
Oil-switches,  automatic,  7 

timing,  216 
Oil-valves,  39 


270 


INDEX 


Open-phase  relay,  nomenclature,  6 

Opens,  testing  for,  254 

Over-    and    under-voltage     relays, 

189 

Overhead  lines,  resistance,  etc.,  102 
Overload,  effect  of,  98 

elementary  protection  from,  133 
release,  9 
relays  on  A.C.,  66 
Condit,  24 

General  Electric,  28,  38,  75 
Westinghouse,  29,  31,  191 
Overvoltage  protection,  64 
release,  13 


Parallel  feeders,  direct  current,  60 
elementary  diagrams,  61,  85 
protection  of,  152 
savings  effected  by  use,  152 
Phantom  loads,  235,  242 
Phase  rotation,  131 

relay,  nomenclature,  5 
Pilot  wire  system,  160 
Plotting  the  results,  244 
Plunger  type  relays,  24 
Condit,  24 
General  Electric,  35 
testing,  227 
vs.  induction,  82 
Westinghouse,  26,  29 
Polarity  directional  relay,   nomen- 
clature, 5 

Polarized  power  directional  relay,  47 
Polyphase      potential     transformer 

groupings,  124 
transformer  protection,  147 
Potential    transformers,     accuracy, 

112 

compensation,  124 
effect  of  frequency,  124 

of  wave  form,  124 
polyphase  groupings,    124, 

125,  126 

temperature  rise,  124 
use  on  overvoltage,  124 


Power  bridge,  262 
Power-directional  relays,  A.C.,  85 

cross-connected,  165,  169 
current    leading    30    de- 
grees, 130 
differential,  96 
double  contact,  170,  172 
early  corrective  features, 

86,  92 

failure  of  early  types,  86 
General  Electric,  93 
iron-clad  type,  87 
latest  developments,  89 
nomenclature,  5 
practical  requirements, 

88 
protection  of  generators, 

149 

of  parallel  feeders,  164 
of  ring  systems,  183 
relay  specifications,  96 
star-delta  connection, 

130 

testing,  248 

used  with  overload  re- 
lays, 95 

Westinghouse,  90 
B.C.,  44 

applications,  44,  55 
Condit,  51 
General  Electric,  49 
storage   battery    protec- 
tion, 58 
Westinghouse    D'Arson- 

val,  46 
polarized,  47 

Precautions  in  testing,  230 
Prime  movers,  failure  of,  59 
Principles  of  operation,    circuit 

breakers,  7 
Condit  overload,  23 

power  directional,  52 
contactor  switch,  72 
cycle  counter,  212 
General  Electric  air  valves, 

36,37 
D.C.  power  directional,  50 


INDEX 


271 


Principles  of  operation,  General 
Electric  induction  over- 
load, 75 

series  overload,  25 
Leeds    and    Northrup  power 

bridge,  263 
overload  release,  9 
overvoltage  release,  13 
protective  relays,  4 
shunt  trip  attachment,  10 
testing  transformers,  221 
underload  release,  11 
under  voltage  release,  12 
Westinghouse  A.C.  tempera- 
ture load,  204 
bellows  overload,  32 
bell-ringing,  199 
cycle  counter,  214 
D'Arsonval,  45 
D.C.  definite  time,  27,  29 
D.C.  temperature,  201 
fault  localizer,  261 
induction,  66 
polarized,  47 
telegraph,  191 
transfer,  209 

Proper  connections  for  3-phase,  120 
Protection,  elementary  overload  dia- 
gram, 133 
generators,  148 
ground,  150 
industrial  plants,  110 
lines,  150 
motors,  132 
networks,  175,  185 
overvoltage,  D.C.,  64 
parallel  feeders,  152 
polyphase  transformers,  147 
radial  systems,  175 
reverse  phase,  192 
ring  systems,  183 
rotary  converters,  139 
storage  batteries,  55 
synchronous  motors,  136 
three-phase  circuits,  134 
transformers,  140 
two-phase  circuits,  134 


Protection,  undercurrent,  64 

undervoltage,  65 
Protective  relays,  1,  3 

nomenclature,  5 

principles  of  operation,  4 
Puncture  test,  221 


Qualifying  terms,  6 
Quick  resetting  air  valves, 


Radial  distribution  system,  30 
network,  182 
parallel  feeders,  181 
protection  of,  175 
Ratio  errors,  113 
Relation  of  various  parts  G.  E.  relay, 

79 

Relay  characteristics,  108 
contacts,  71 
specifications,  overload,  83 

power  directional,  96 
switches,  17,  205 

testing,  223 
Relays  and  transformers    required, 

133 

Releases  for  circuit  breakers,  7 
overload,  9 
overvoltage,  13 
shunt  trip,  9 
underload,  10 
undervoltage,  12 

Requirements,  practical  power  direc- 
tional relay,  88 

Resistance,  overhead  lines,  102 
three-conductor  cable,  103 
units,  233 

Results,  curve  of,  246 
plotting,  244 
tables  of,  245 
Reverse  current  relays,   see  Power 

directional  relays, 
phase,  protection  against,  192 
relays,  193 
testing,  249 


272 


INDEX 


Ring  systems,   alternating  current, 
175 

direct  current,  62 

more  than  one  source,  185 

parallel  feeders,  185 

protection  of,  183 

Rotary  converters,  protection  of,  139 
Rotation  of  phases,  131 

S 

Savings,  use  of  parallel  feeders,  152 
Schematic  diagrams,  also  see  Princi- 
ples of  operation, 
bell  ringing  relays,  199 
circuit  breakers,  9 
Condit     D.C.     power    direc- 
tional, 52 

plunger  overload,  24 
cycle-counter,  214 
General  Electric,  D.C.  power 

directional,  51 
induction  overload,  25 
strap  wound  overload,  28 
unit  type,  35 
insulation  test  outfit,  221 
shunt  trip,  9 
underload  release,  1 1 
undervoltage  release,  12 
Westinghouse   bellows   over- 
load, 32 

D.C.  power  directional,  47 
definite  time,  27,  29 
induction  overload,  67 
overload  telegraph,  191 
Secondary  load,  effect  of,  115     , 
Semi-automatic  circuit  breaker,  7 
Series  transformers,  112 
Series-trip,  18 

objections  to,  18 
Series-type  relays,  G.  E.  overload,  28 

testing,  223 

Service  restoring  relays,  194 
Short  circuits,   accurately  locating, 

256 

calculation  of  current,  101 
effect  of  unbalanced,  106 
nature  of,  100 


Short  circuits,  testing  for,  254 

vector  diagrams,  107 
Shunt  transformers,  112,  122 
Shunt- trip,  9 

attachment,  10 
elementary  diagram,  17 
schematic  diagram,  9 
Signal  relay,  nomenclature,  6 
Single-phase  groupings,  117 
Slide  resistor,  235 
Solenoid  plunger  relays,  24 
overload,  25 
voltage,  190 
vs.  induction,  82 
Source  for  testing,  232 
Sources,  trip  circuit,  16 
Specifications,  overload  relay,  83 

power  directional,  96 
Split  conductor  system,  158 
Standards  for  testing,  236 
Standby  batteries,  58 
Star-delta  transformer  banks,   pro- 
tection of,  147 
•connection,  130 
States  Co.,  phantom  loads,  236 
Storage  battery,  protection,  55 

testing,  225 

Strap  wound,  horizontal  bus,  53 
plunger  overload,  24 
power  directional,  51 
schematic  diagram,  52 
vertical  bus,  53 
relays,  Condit,  51 

General  Electric,  25,  48 
Stray  fields,  effect  of,  92 
Supervisory  circuit,  74 
Switches,  automatic  oil,  7 
auxiliary  relay,  17 
contactor,  71,  90 
relay,  205 
testing,  223 

Synchronous  motors,  protection  of, 
136 


Table  showing   comparison   Z   and 
delta,  121 


INDEX 


273 


Table  of  results,  245 
Tables  and  curves,  228 
Tap  plates,   G.  E.  induction  over- 
load, 78 

Westinghouse  induction  over- 
load, 69 
Temperature,  compensating  screw  in 

G.  E.,  81 

load  relays,  138,  203 
relays,  200 

nomenclature,  5 
testing,  249 

rise  of  current  transformer,  116 
of  potential  transformer,  124 
Testing,  actual,  239 
A.C.  relays,  231 
clips,  226 
D.C.  relays,  220 
equipment,    storage    batteries, 

225 

for  grounds,  221,  254 
opens,  254 
shorts,  254 

heavy  current  relays,  241 
loads,  223,  232 
millivolt  relay,  226 
phantom  load,  242 
plunger  series  relays,  227 
power  directional  relays,  248 
precautions,  230 
relays,  225 
relay  switches,  223 
reverse  phase  relays,  249 
source,  232 
standards,  236 
temperature  relays,  249 
time  limit  relays,  228 
voltage  relays,  246 
Three-phase  current  transformer 

groupings,  123 

Three-wire  current  transformer  dia- 
grams, 117 

Through  type  transformers,  113 
Time  delays,  16,  19 
definite,  21 
how  obtained,  23 
inverse,  20 

18 


Time  delays,  definite,  21 
limit  relays,  29 

testing,  228 
Time-load  curves,  77 
fuse,  20 

G.  E.  bellows  overload,  20 
definite  bellows,  21 
induction  overload,  22,  180 
Westinghouse   induction   over- 
load, 22,  179 
inverse  bellows,  33 
overload  bellows,  178 
Time  settings,  G.  E.  induction  over- 
load, 78 

Timing  circuit  breakers,  216 
closing  relays,  215 
opening  relays,  216 
the  relay,  238 

with  cycle  counters,  212 
Torque  compensators,  69,  92 
Transfer  relays,  19,  208 
Transformer  banks,  protection,  143 
protection,  140 
accuracy,  124 
Transformers,  and  relays  required, 

133 

compensation  in  potential,  124 
constants,  105 
effect  of  frequency,  124 

of  wave  form,  124 
groupings,  123,  124,  125,  126 
instrument,  112 
load  on,  83,  127 
magnetization  of  core,  114 
protection  of,  132 
of  polyphase,  147 
of  star-delta  banks,  147 
temperature  rise,  124 
use  on  overvoltage,  124 
Westinghouse  voltage,  247 
Trip  circuits,  16,  238 
Trip-free  circuit  breaker,  7 

relay,  nomenclature,  6 
Tripping  current,  adjustment,  79 
Two-ammeter  method,  259 
Typical  layout,  217 

time  of  circuit  breakers,  180 


274 


INDEX 


U 


Unbalanced  short  circuits,  effect  of, 

106 
Undercurrent  protection,  64 

relays,  191 

Underload  release,  10 
Undervoltage,   and    excess    current 

protection,  187 
protection,  65 
relays,  189 
release,  12 

Unit  type  relays,  28,  35 
Use   of   potential   transformers   on 
overvoltage,  124 


Valves,  quick  resetting,  36 

Various  parts,  relation  of,  in  G.  E. 

relay,  79 

Z-connections,  122 
Vector  diagrams,  short  circuits,  107 
three-phase     current     trans- 
former groupings,  123 
potential   transformer 

groupings,  125 

two-phase   current  trans- 
former groupings,  118 
potential    transformer 

groupings,  125 
Z-connection,  120 
Vertical  bus  relays,  25,  50 
Voltage  relays,  189 

nomenclature,  5 


Voltage  relays,  testing,  246 

transformers,  112,  122,  247 

W 

Ward  Leonard  resistance  unite,  233 
Watt  relay,  nomenclature,  5 
Westinghouse  fault  localizer,  260 
relays,  A.C.  power   directional, 

90 

bellows  overload,  29,  32 
bell-ringing,  199 
cycle  counter,  213 
D.C.  power  directional  D'Ar- 

sonval,  46 
polarized,  47 
high  tension,  211 
induction  overload,  67 
temperature  load,  203 
voltage,  189 
interlocking,  96 
multicontact,  207 
obsolete  definite  time,  26 
overload  telegraph,  191 
relay  switch,  206 
reverse  phase,  193 
service  restoring,  194 
transfer,  19,  208 
What  are  protective  relays,  1 
Wrong  connections  on  three  phase, 
119 


Z-connections,  advantages  of,  120 
various,  122 


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